I should really get back to the collection. Beating Bloodlines with Eric, Kid Dracula, CV2 and 3. I've got a lot to do. The two NES games I'm missing are the most harsh to play, because CV3 frustrates me to no end despite its qualities (maybe I'm better off playing the Jap version now that it's available) and CV2 is just an annoying trial and error if you're completely new to the game like me.
But now I've got Bloodstained to finish which I want to do asap and I've been playing The Witcher 2 for the first time which is quite the time sink.
Just the other day I finally completed the whole collection. My only regret is using what turned out to be an unnecessary save state to learn the roller coaster part of Kid Dracula. Because the very next time, rather than practicing it, I nailed it. (I'll probably go back and do that section in one run, just to clear things up for myself.) Other than that slight concession, I beat the whole collection without any methods other than what the games originally had in them (i.e. passwords and such). Apparently, on Steam, it says only 2.6% of users have completed the whole collection. I wonder if that's because of the challenge level, or just general disinterest.
Kid Dracula is deceptively challenging, and the last stage, which I beat fair and square, is downright brutal, having no checkpoints. There is a certain charm to Kid Dracula and its occasional Castlevania references, and it has some very ambitious ideas and visuals baked into it for an NES game, but it still feels a little odd in the collection to me.
I've started to play around with the Hard Mode in Castlevania III and the Expert Mode in Bloodlines (with John), but haven't committed to either yet. (I'm having trouble getting that extra life where you swing between multiple conveyor belts in the Munitions Factory that
tav7623 noted.)
Castlevania III is tough to learn, but after years of playing it, I find nearly all of it to be an average level of difficulty now--no worse than, say, Castlevania 1. So, having an easier version of it is neat (especially with the new gameplay and soundtrack quirks), but it's sort of too late for me. I've been trained in on the harder version to the point where it doesn't bother me much. (The biggest enduring hurdle of potential frustration/mishaps being the doppelganger fight and water area preceding it, probably...Oh, and the rematch against the upgraded Skull Knight, if I take that path. Okay...and Death's first form can be tricky, too.) The fact that I had to play through it four times for the collection, covering all the stages and/or characters, means I may have had my fill of CVIII for a little while...
The tricks I've learned over the years to mastering it are pay attention to your subweapon, your surroundings, and the boss patterns. I guess that seems obvious enough, but when I was younger, I sort of thought a bunch of it was all random. However, there is often logic to these things. Some boss fights have platforms that a particular character can exploit, some bosses are especially weak to a given subweapon, and all of the bosses have holes in their attacks and defenses if you study them. (Trying to beat Dracula's final form without an axe would be a nightmare, I feel, and for example, with Death's second form, it took me a long time to realize you can consistently walk under him and get around him if you time it right.) It's the same for the stage layout and enemy placement. And the stages are generally fairly generous in giving you multiple times to get the best subweapon for a given job. (Before the Death fight, they give you a choice of any weapon, and a Double Shot for the cross, if you so choose. I largely prefer the holy water, though.) The stage that used to be the worst--the one with the cliffside falling blocks that is on the longest path and has basically four bosses--is now totally doable and doesn't intimate me much at all. I think I've said this before, but when first learning the game, I always recommend Grant (for his agility/speed) or Sypha (for her attack strength). I still have found only an occasional benefit to Alucard, despite the novelty of his bat transformation. But really, when push comes to shove, I often use Trevor, because he's the most balanced, which made the Trevor-only run lot simpler in some ways than I would have thought.
In addition to the character choice, though, is the decision to always take the path that leads to Sypha and the pirate ship for a first learning run. That is by far the easiest and shortest path to victory*. (Since the collection makes you play through all character combinations for achievements, I went through all of the main stage combinations on different runs with different characters that I thought would be the most interesting and/or helpful for some stages--it made each playthrough fresher. It's funny how the game sort of tricks you, though. Like, there's a fork that continues to a third part of Alucard's cave or redirects you to Atlantis, and being redirected will actually shave a [unique but additional] stage off of your journey rather than going straight forward. On the other hand, sometimes going to see some of the more exotic sites also causes you to have some more challenging stages and a longer path overall.)
I'm still amazed how many ways you can go through this game, and how it doesn't feel as artificial/awkward as how Rondo does it. Much as I love the 16-bit era and the unique vibe of Simon's Quest, I think this is why CVIII still muscles its way into my number-one spot for the series. Several years ago, for an "on-the-back-burner" project, I started to map out the diverse quest of Castlevania III. Other things came up, so it never got done, but here is an in-progress map piece that gives an idea of how unique CVIII is and how many ways it can be played. (I never double-checked the math, and don't feel like doing so now, but it's in the ballpark, I believe--unless I totally crossed something up.) But basically, if I recall correctly, any time there was a stage transition that led to an alternate boss, I counted it as a stage, and then on that grouping of stages, I counted what characters you could complete and/or transition to on that path with on one's first playthrough.
*Looking at my own chart here, I see that, technically, if you go get Grant, the pirate ship route is not necessarily the shortest; and also, that the path with Alucard can be tied for the shortest...though has tougher stages mixed in and no direct access to Grant and no access at all to Sypha. Seems it's a little more complex than I realized...
On the topic of the Japanese versions' difficulty/game balance...The cross subweapon may not be as strategically useful in spots as the axe in Belmont's Revenge, but in most situations, having played with it now for just a little bit, it seems way overpowered. It's just so big and hits twice and covers a far-reaching area. (It also seems like overkill with the fire-spitting whip.) I can see why it might have been swapped out, though it may have just been a censorship thing. In other words, it's very cool, but it might break parts of the game and basically make you just want to use it, whereas the axe and holy water produce more of a legitimate choice--especially, again, with the flame-shooting whip.
I'll close by mentioning that I got the middle ending for Simon's Quest. (I think I took 11 or 12 in-game days.) Not sure if I've gotten that one before or not, but I know I've never gotten the best ending (I think you need to beat it in 7 in-game days or less). I can see how the best ending could be done now, though. It's weird, but you'd basically skip over a lot of the extra content/side quests (that are sort of the things that make the game great, in my mind), and just blast through the main quest. I kind of don't like it when games do that.
Anyway, I found some sequence-breaking tricks this time through to get some powerful weapons early, which was fun, but it still didn't garner me the best ending. I wrote a lot about the game in this thread on June 4, but reassessing it after beating it now, I think that the biggest issue with Simon's Quest, conceptually, is that most of the neat things you get in side quests and for extra exploration are superfluous. You either get them too late in the game to be useful, they eat up too much currency when you need it elsewhere, or you get a more powerful replacement not long after getting the base model and don't have to go out of your way for it. So, that's a problem, especially when getting some of these extra things eats into your chance at the best ending.
Off the top of my head, I think the three crystals, the holy water, Dracula's remains, laurels, wooden stakes, and the cross are all you need to beat the game. You probably could even get away with not upgrading your whip all the way, though that'd be more trouble than it's worth. But not counting that, that leaves at least seven power-up items/weapons that are entirely optional...and that's not counting the many hidden riddle books. Even so, I still love that the content is there, and it feels good finding and collecting it all--in other words, it adds to the "quest" without being a chore. Like, if you're going to get a weapon you don't need, getting it should at least be a surprise and it should at least have a notably unique property that other weapons don't. (That's what makes it a shame that they don't come too much in handy, despite their novelty.)
Sorry to hear about your frustration with Bloodstained. It sort of sounds like how frustrated I was with Zelda: Breath of the Wild, where I didn't feel it was utilizing its elements to the max, but it had become something of a media darling.