I don't think even the original castlevania trilogy has this much bullshit level design in it. I get that you're going for that "nintendo hard" feeling, but towards the end of the game it gets more frustrating than fun. There is no reason that I should be dying 20+ times in a row to the same boss.
I'm mainly salty about the last boss, which again, took me at least 20 attempts to beat. The boss is more luck than skill. Wanna know why? Let's take a look at his attack patterns. To start, you have the guy spamming attacks at either end of the screen, all of which deal insane knockback, and the longer he is alive, the more shit gets spammed onto the screen. The only way I've found to deal with this phase without tanking your health to near zero is to spam the hourglass and wail on him relentlessly.
In the second phase, he spams attacks from every other boss in the game, which is cute and all, but when you can't predict what attack he will use, there are problems that arise. For example, the tongue attack from the previous level's boss will hit you if you're standing too close. You have to jump to avoid it. But it comes out so fast, and with no warning, that you have to pre-emptively jump JUST IN CASE he does it. The problem with that is that he could also just use the bone attack, which will hit you if you jump. So you can stand still and get hit (or not get hit) based on RNG, or you can jump and get hit (or not get hit) based on RNG. It's an unpredictable clusterfuck and the only way to beat him is to get lucky.
That's my problem with this game: it's unpredictable, and it's not because it's RNG, nor for a lack of effort on my part. It's because the enemies do not telegraph all of their attacks. You can't use the fact that it's a love letter to castlevania as an excuse for this design flaw. Unfair =/= hard. You had the sense to avoid resetting the player to the beginning of the game after an arbitrary number of deaths; you should have also animated telegraphs into the enemies' attacks. That's not hard. That's just bad game design.
On an unrelated note, the achievements are broken. I only got the pumpking achievement despite beating the game. See screenshots. You should look into that.
Edit: I originally said I died 45 times to the boss, but actually I wasn't counting. 45 was just my total death count and I was salty, so I just said 45. Actual death count was probably closer to 20, hence the edit to 20+