The weapon felt so satisfying to use and I particularly liked the brief recharge sound it makes when you stop shooting. Platforming also felt good and both regular jumps and double jumps felt quick and snappy, but I would’ve liked to have even one more movement ability, such as a dash (maybe it was actually there as a secret?), so that I could’ve moved faster through already explored areas.
Enemies were weird. The spiked creatures could’ve benefited greatly from having a brief death animation to signal their imminent explosion, while the plants’ projectiles needed to either be a bit slower or better signaled. A dash/dodge ability would’ve been useful here, too. However, with health upgrades being abundant, combat never actually felt too unfair.
The ability to damage slimes and the one that renders you immune to the toxic fumes felt a bit like glorified keys: they basically only allowed to access new areas without contributing to the overall gameplay (this is actually a very common trap to fall into, considering that many other metroidvanias have done this, too).
I may have sounded harsher than I actually intended to. In the end, I enjoyed playing through this and I think you really nailed the early PS2/GC era atmosphere. I know you said you don’t want to continue developing your entry, but I honestly think this is a great foundation to build upon and could really shine with improved enemies and more platforming abilities!
PS: Just FYI, even though there’s no native Linux build, the Windows one works perfectly under Wine/Proton!