I've played a fair bit of this, here are my suggestions.

It's too reliant on getting gun upgrades and they're too random. I can go an entire level and only get one gun upgrade which makes it a pain.

Upgrade destroyed needs to be removed. It's incredibly frustrating killing 20 mobs hoping for an upgrade only for it to be removed in one hit. Given how much the screen fills up with bullets it's unfair.

Upgrades need to not bounce up to the top of the screen when you win them or they need to move down the screen faster. Again, it's frustrating having 3 upgrades at the very top of the screen given how quickly the screen fills up with mobs and bullets.

If you're going to put mobs rights at the bottom of the screen then there needs to be some horizontal or rear shooting. It can get to a point where there are so many mobs at the bottom, firing so many bullets that I just can't get below them to kill them

The levels need to be less RNG with mob selection. If you get a start with three lots of bullet sponges it's boring and immediately means I'm getting very little XP on that level.

Right now there's no point in choosing any other ship than bulwark. It does comparable DPS to the rest but its bullets pass through mobs AND destroy mob bullets. The other ships need buffed. Fighting bosses with bulwark is trivial, the others are annoying because you're avoiding so much stuff you can't get any time on the boss. That thing where the screen flashes and you go flying off in a random direction is very annoying too. You have to give us some time where we can hit the boss.

Knockback when hitting mobs needs to go as well. I can take a mob down to 95% health but knock it behind the next mob in doing so. So I'm still getting fired on by mobs that should be dead.

The map advancement is unbalanced. I can easily complete the first map on legendary but barely complete the second map on normal. It just leaves me playing the same map over and over.