Too easy?

Not too easy, not easy at all in my first sight

This is sort of a non-complaint really: any game where you can engage in tedious levelling activities to become more powerful against (non-auto-scaling) enemies can be made "easier" by a trade-off of time/interesting activities against "skill". 
That doesn't make them "easy", as the base level of challenge is what determines that.

Bloodstained is for the most part harder than SOTN. If you play on hard which I agree should be available from the start it's much harder. The first boss in ROTN is so much harder than Slogra and Gibbon. The way the games are made, being RPG's, make it nearly impossible to be to hard because it all depends on how leveled up you are by the time you get to different places. Inverted Castle was pretty hard but everything before then is a cakewalk in SOTN. Nightmare mode flat out destroys SOTN but it also isn't the same experience since you can't level up.If you do no grinding in ROTN it gets hard even on normal.

Nightmare?

Maybe I'm missing something...you were saying that even Hard was too easy and the game would benefit from a very hard mode (which is Nightmare), and you haven't tried it, but you don't see how you can progress in it without gaining levels - insinuating that it is too hard. Is that right?

If so...that sounds like Nightmare should be for you the sweet spot for the game otherwise being too easy on hard mode. However, if what you want is a game that is hard and yet works and is balanced for the notion that you don't ever want to level up, that would...probably be Hard itself or you're being very picky about it!

What you might also benefit from then is Classic mode when that hits as it would have no leveling at all, but it seems rather like you want something more like SotN's difficulty curve, but that seems contrary too because that game is pretty easy. idk

lol

Though amusingly enough, that's the very thing most repeated beforehand they didn't want to see in harder difficulties. Anything but a "lazy hike in HP values".

It's more or less though inevitable in a game like this - your character will just be too powerful for much of anything else to have a meaningful effect.