It gets really chaotic and hard once enemy portals start appearing everywhere. This is fun and very reminiscent of Geometry Wars.
The screen shake is done well. Feels impactful but not disruptive.
I like having the dash as a way to bypass an incoming enemy swarm.
The player's weapon doesn't feel wide enough. Starting with a twin shot like in Geometry wars would feel better. It also doesn't feel like it grows in power from upgrades very quickly. By the time I'm being swarmed by lots of enemies, the weapon feels inadequate.
I'm mostly only worrying about the red enemies, since they're the only ones that target the player. It would be more interesting to have a wider variety of enemies to shoot at:
You could make the portals destructible but durable, giving the player a target to hunt down rather than always trying to stay away.
You could add an enemy that starts off like the greens, but seeks the player after hitting a wall.
Or an enemy that explodes into a scatter of greens when it dies.
Or an enemy that bounces like a green, but periodically spawns a small number of reds.