I'm not entirely sure what to think about this. I know Misery, that you want to emphasize more ont he bullet hell aspect of the game and that players have to last a very long time in a cramped boss room until the enemy finally goes down. However, you come fromt he bullet hell grenre which plays mostly different. Even if you tied your experiences into this game, I never saw it as "fully fledged" bullet hell and I'm pretty sure people that see this game at first glance will say it's a rogue-like/Isaac-like game. But people except a different concept of this, they don't except long fights where you have to dodge tons of stuff, they except that you can take hits or wear down the enemies fast enough so you can go on.
For me this was always the case, even if I'm nor very good at dodging, going the other way, pure damage or pure health, helped me to overcome my weakness. However, it seems you both (Pepisolo and you) try to negate this more with each patch and make enemies last longer, nerf damage here and there. Pleas ekeep in mind, not everyone that comes to this game sees it as bullet hell. That's why I think the mor ebullet hell like stuff should be on the higher difficulties (including hp/damage scale) while at nomal and lower diffculties you still should have a good casual experience.
I will first test this out and give a final statement if this is true, however I just wanted to warn you guys. The most complain about SR I hear from my friends (especially one who REALLY loved Isaac but hates SR) is that the game feels too much like bullet hell or your damage is too low and bosses take too long. I know that you put much effort in the bosses and want players too see them long enough but the thing is, not only can they take a lot of damage, they DEAl also a lot of damage. If you cut damage on players, do it for bosses as well.
The reason the changes are made is because for quite some time now, the typical run for players is that they hit a certain point in the game... say, right around stage 4 or 5.... and they become so powerful that there may as well not even BE anymore stages. Terminus, for example, the most freaking tanky thing in the game with a huuuuuuuuge amount of HP (this being BEFORE any changes), could be destroyed so fast that it NEVER FIRED A SHOT. Considering the nature of it's shields, this should have been impossible, but the player's damage was so astonishingly fast that the shields couldn't even form.
However, on the other end of things, if for some reason you specifically DECIDED to go a very defensive build instead, suddenly the game would take 10000 years to finish because your damage would be so low.
So you had this huuuuuuuuge gap between the two; this is probably why your friend there says the bosses take too long, because he's probably using a defensive build; whereas with an offensive build (which is just as easy, if not easier to accomplish) the fights are over so fast that they aren't even "fights". That's how huge that gap is, going from "this is taking absolutely forever" to "What? That was supposed to be a boss? I thought it was a crash test dummy".
In a game like Isaac, this actually works, because Isaac is kinda built around those wild changes. But this game isn't like that. This is a bit closer to something like Gungeon actually, where the gap needs to very specifically NOT be super huge. That's not to say that OP builds should never be possible; in a game like this, having the possibility be there is part of the fun. But it's supposed to be a relatively rare thing outside of Isaac. Here though, it was happening basically every single time if the player decided to even somewhat focus on damage.
The game isn't going to be made into something where everything takes forever, I can tell you that much. It doesn't matter what the damn bosses (or whatever) are like, or wether I personally made them or not.... if they take too long to go down, I start to dislike them. No patience, remember. Gungeon does that bit, the bit where the bosses take freaking forever, and it absolutely drives me crazy. I'm not wanting the bosses to take super long. The ONE exception might be Terminus.... sort of. It has 7 freaking forms, it is the one fight that is supposed to be long. But if every boss in the game was like Gungeon's bosses, taking forever to kill, it'd drive me crazy.
Currently, on my end, much of the changes being made is enemy and boss HP being lowered. I can tell you it's soooooooooo much easier to get these set now; having tried different types of builds with the White Gloss, there's no giant gap of doom anymore. I don't have to GUESS as much at these bloody numbers, even despite health scaling.
Also, enemy damage: It's hard to say what that'll be like. However, one way or another, nobody really liked the damage scaling for enemies. That ALL enemies just did more damage in the late-game simply because the engine says so was kinda annoying. NOW, it can actually be set. If an enemy is supposed to be a weakling (or maybe it first shots that are weak, but fires a ton of them), it can be set to do only 1 damage without the game going "well, no, see, this is floor 6, so that 1 damage is now 5 damage". If it's supposed to be this super-dangerous thing yet without firing all that many shots, maybe it's higher than normal. Late-game enemies will tend to have more damage, but I don't think it's going to end up being as high as it was before, now that attrition is possible. It's going to be more based on enemy roles now. Boss/miniboss damage, for the time being, is likely to remain consistent throughout the game (instead of rising). I don't see all that much reason to change those guys up so much, for now. Which means that the late-game bosses wont be hitting as hard as they are in the version you have.
One way or another, there is a good chance that the game might get harder here, as I said. But don't worry about it going too far... keep in mind this topic is moreso just pointing out how the whole thing is RIGHT NOW. AKA, in it's unfinished state. I posted this topic in case anyone (such as yourself) had any comments about all of this that might lead to further changes or anything.
Also for the record, I don't see this game as "fully fledged" bullet-hell either. Much as I love that sort of thing, to me this is an Isaac-style game above all else. If I were really treating this as a pure bullet-hell game, well.... I think most players would find it impossible to beat even on Normal mode.
Now that being said, if you should find anything in the game, anything at all, that seems to really stand out as being "way too much" in terms of difficulty, then please do let me know, so I can have a look at it. Sometimes we get things that are giving players ALOT of trouble... but they really aren't supposed to. I tell ya, it's tough to get that right.
Of course some things that do that absolutely are meant to, hah...