First off, really cool game! It controls well, has a good atmosphere, and the music is amazing! The gameplay while fun I think can benefit greatly from a few minor tweaks.
Before anything though, the game is currently incredibly difficult. I had to turn down enemy spawns to 70% to even get past the 1st quest. And subsequently turned it down to 50% to beat the last quest.
This game wants you to play as fast as possible but there isn't enough incentive for it (other than the core % increasing faster). Having more mechanics in place that reward players for playing at high speeds would really help out. Something like increasing size based off velocity to make it easier to take out enemies, or maybe if you do kill an enemy at a high speed it heals your core.
Another problem I had was accidentally entering the core and leaving a level before I had completed my current quest, from what I can tell the quests are linear so I think it would just make sense to only open the core when the current quest is complete.
This is all my opinion obviously but I really think this game can be amazing with just a couple changes :)
Excited to see what you do next!