Having a lot of fun with the game, it's already very good!
I have some feedback that I think would improve the experience a lot: Enemies really need some kind of death animation, particles, a little 'pop' animation, something at least. As it is right now the game feels super polished in the menus, but during gameplay stuff like lack of feedback upon enemy death and that enemies completely ignore walls is really jarring. It feels like a huge blemish on an otherwise super-polished product!
You also definitely need some kind of critical hit feedback. Enemies don't have health bars and they don't have pushback upon being hit, so there is almost no way for a player to know whether a hit was a crit or not. This makes the critical hit upgrades completely interchangeable with the extra damage one, as all you really know is "I'm probably doing a bit more damage on average".