The game is surprisingly good and fun! The tension is very high, the pacing is extremely fast, and the music is very fitting too. The difficulty is quite high though but we'll talk about it later. Generally, I think you have done a great job making this game!
Now down to the criticism (You've asked for it and it may be longer than usual mind you). One of the most blatant things I would want to point out is difficulty. Be get me wrong I love difficult games, but quite a few things here feel a bit unfair. For instance, the fireball guy throws quick fireballs without any/super minor telegraphing, which leads to cases where it's quite humanly impossible to react when you see it, especially when paired with the fact that you need to face the fireball to even try to parry it. My suggestion on this will be to do some obvious telegraphing animation by the fireball guys and allow the player to react accordingly. This also applies to the melee guys who also dash into you very unexpectedly quickly. I had to admit I died to them way more times than even the final boss. Telegraphing is key in doing fast-pacing games.
Speaking of parrying, I love the idea of parrying, but I feel like the parrying window is quite short, and I strongly suggest not using the middle mouse button, as that is the worst button (slow, unstable as it a wheel, people are not used to pressing middle button) to press on any mouse I've encountered. Any key will be better than that. Btw melee is almost useless in the game, low damage low range, against any enemy it's never worth it to trade health by meleeing as your health is so low and you'll be guaranteed to take more damage than needed. Also, why the heck melee guys can dash through walls? That's not fair at all!
Nothing to say about the summoners. This is an example of telegraphing done right, but I'd say given the player's moving speed, their attack is a bit too quick. I had to say that the final two rooms before the boss were quite hard, but that's due to the fact that the melee and fireball guys exist. The rooms here are way too small for any effective evading, and it's feeling like up to rng if I get damaged too much or not.
Last but not least, the final boss. Firstly I somehow managed to parry once only, the graphic of that bullet doesn't convey that it's parryable, so either it's a mistake or you should reuse the fireballs instead. I'd say he's probably easier to deal with than the melee guys to be honest, but I guess it's just me getting good at the game. He's a bit annoying after a few tries, and the profanity is getting a bit too much, and I am saying that as a binding of isaac enjoyer.
But anyway, this is quite a great game, especially by a solo jammer. I'd say you've done a fantastic job!