Having letters on the upgrades is a big improvement. I got all the unlocks and got down a few depths.
Some more feedbacks / suggestions
+ I think the default weapon should be the pistol, the sub-machine gun sucks way too much early on, and it's just not fun hoping you get lucky with the spread.
+ Consider adding a "first one's free" upgrade at depth, or even just on the very first
+ It'd be very helpful / cool if you could see the weapons stats [before]->[after] when hovering with a letter upgrade.
+ When you hover over an item the available slots could light up, maybe depending on letter compatibility.
+ The notification telling you to buy unlocks is the most prominent thing on the screen, before you can even buy an upgrade
+ Gaining biomass is very subtle. Could use a sound effect / animation to indicate when it's happening.
+ Wish you could sell / delete weapon upgrades.
+ Wish you could freeze an upgrade or buy it but not equip until you unlock (or activate?) the slot. There's just so many reasons an upgrade is probably irrelevant or unusable, it could use a mitigation imo.
Game is super rad. Best of luck and congrats on the release :)