Great game, had a lot of fun playing it!
The gameplay is great,and very satisfying.
Feedback:
-You did mention that you have not included any sounds, if you are planing on improving this game you should totaly add some sounds when hitting an enemy this would make this game even more satisfying.
-Make it more aparent that you have to hit the blue gems in some of the enemys, took me a while to figure that out you could for example add some red outline with a shader to gems that where not collected(e.g. are in an enemy)
-The map size is fine, but you should maybe add some feedback for the player so he knows that he is reaching the map limit
-Enemy variety, this is something you probably are/plan to work on, if not, some more enemys especialy in the late game would be great.
-The powerups are good, they feel very rewarding.
-Make the time slowdown when hitting an enemy slow down less and shorter, for me the slowdown felt too much like an fps drop beforre i got used to it.
-And finaly, enemy spawning, I got to about 45 gems, and at this point the enemys were very fast, almost impossible to dodge(that's a skill issue on my side tbh) but maybe change the spawning system to spawn enemys based on gem count, this would allow you to balance the enemy damage/speed/spawn rate with the players estimated power.
That's a lot of feedback, but don't get me wrong I realy enjoyed your game, those are all things I noticed that could be improved.