As an avid Shmup player, I really love the core mechanic of swapping between ships to pick up items.
I could see this idea being expanded upon and leading to a very fun game filled with tense movement decisions (what if, for example, each ship had a different weapon you had to adapt to when swapping to pick up items...)
As it stands out, the game is really fun and I had a good time discovering the abilities of each ship, I'm also impressed by the amount of content you've put in, even including a boss battle!