Ok, listen. I know it, you know it: the game is pretty darn cool. Super-duper cool. So as much as I'd love to spray you all with a hot load of the same ol' praise you've probably heard a million times, I'll spare you that and get right to the nitty-gritty: feedback.
As said, I'm overly very positive on the game, so most of this is just nitpicks: I didn't find any serious dealbreakers. Forgive my verbosity:
*I found myself surprised at how long levels felt. For example, when I played the second level, it felt like it just kept going on forever as it all devolved into a mish-mash of constant enemies. It was fun, but the fact that I noticed it did make me feel like it was getting a bit repetitive. Might be due to a few things: the game might've introduced too much enemy variety too early and then plateaued, or maybe the game wasn't having enough unique events happen (like those moments where suddenly a big circle of enemies encloses you).
*As cool as the reboot process after a death was the first time, is there a way on subsequent runs we can just press a button to fast-forward through all the heartbeat ponging stuff and just get to the stats and ability to go back to menu?
*The exp pickups being numbers created from damage is novel, but I felt like it was kind of less than ideal. For one, it makes it very difficult to be able to tell the damage your individual weapons are doing, which makes it both confusing to determine how strong they are at first and makes it less apparent and thus less satisfying to see them grow bigger from upgrades. Second, I found myself regularly forgetting to pick them up since they both just blend into the background and aren't very distinguishable from each other in intensity as something like blue/red/yellow gems are, making it unclear what to prioritize to pick up (is there a big number in this sea of numbers that's worth going for or are they just a bunch of worthless 1's made from scratches of my laser? hard to tell with peripheral vision in this chaos).
*Skills points...bit unsure on some of these. For example, some of them feel like they should be weapon pickups instead, like the scream pushing back enemies when you get damaged. In general, I'm also just not sure if they are even necessary as a system: I guess metaprogression can feel nice but sometimes it can be annoying that you feel like you can't actually play a "fair" game until you've spent time and grinded all the unlocks, like why aren't they just part of your base toolkit?
*By default I feel like the game put the sound effects (specifically weapon hit feedback) too soft and the music too loud. Maybe you guys got sick of hearing the weapons drone on and on during dev so you turned it down, and maybe I'll get sick of it eventually and wish I kept it to your settings, but for right now, I feel like its more satisfying to hear the hit feedback in a louder fashion.
*The story is very cool and mysterious, but I almost felt like the game gave too much away in the first memory? Lore is an awesome thing to collect and I was looking forward to playing farther to get more of the mystery unlocked, but I practically feel like I already understand everything from just the first boss memory so my drive is a bit diminished as what else is there to strive for? I could be underestimating the carrots you have left to dangle, however, if you pardon my pun.
*Some boss phases can be a real pain for certain loadouts: for example when the rabbit boss flies to the edge of the screen and is shooting circles at you, I felt like a chump since my loadout was all melee and I couldn't reach him even if I kept moving towards him. Luckily he has a phase after that where melee is good, but just be aware of this going forward in designing. Is it my fault I didn't diversify my loadout, or is it games fault it didn't design the boss or ensure I get provided enough versatile weapons? I think it's fine, but food for thought.
*As both a programmer and a lover of mystique, I should've loved the weapon descriptions done through psuedocode, but in reality I just found myself confused and annoyed. You can eventually learn what the weapons do through experimentation, though having said that I actually still don't know what a lot of stuff does for certain. Maybe a bit more clarity would be nice while still preserving the codey theme?
*You probably already know it, but this is just another Vampire Survivors clone (or maybe Risk of Rain clone, or whatever else if you keep going backwards) so as stylish as it is, it is probably gonna get a lot of flak for being in an incredibly saturated market. I certainly think this is doing a lot better by having a very unique style and having a more powerful story and mystique about it, but still, whatever else you can do to differentiate it, do it.
*I liked the enemies that sleep until you hit them, whereupon then they come running at you, but I think when I had a laser I never saw them asleep again: wonder if the range on the laser is too much and creates off-screen aggro?
*Apologies if you watch my video of me playing: I did way better once I played it off-recording, haha.
Best of luck on your continued development!
(Note: all of this is from playing the Steam demo as you advised)