These mechanics are pretty fun. Swapping between the formations is a very interesting way  to handle potential different types of enemies or the number of enemies in a wave, even the amount of bullets. I personally have a hard time tracking a lot of things happening on the screen at once, especially if there are lots of small things to watch out for. So I kept my formation tight in the V shape to gather many ships in my formation and destroy anything ahead of the arrow I formed. But spreading that formation of 20+ ships out beyond the V formation  was too much for me to track when i had all the red bullets flying at me while trying to make sure I destroyed enemies, even though the line that formed between the ships would destroy bullets, aside from what I thought was huge missile (but was actually a kamikaze enemy) that would blow up a group of my ships from out of my formation (just my skill issue not  being able to avoid it~ lol). I would consider your bullet readability. I think it is hard to see the player bullets versus the obvious enemy bullets. Also make the kamikaze enemy more obvious, like, maybe give it a flashing effect or something that draws focus to it so the player has more time to react to something more threatening. I did not go beyond wave 3, as if I lost too many of my ships, I became super vulnerable at that point by the amount of enemies and bullets that exist in gameplay that I have no protection or skill to not die.

Maybe consider adding a visual indicator of player lives... even though it becomes obvious when you play multiple times, that you have 3 lives. If my ships did not have shields... I would never make it to Wave 3. lol

I really enjoyed the formation "connection" mechanic. This was my favorite part of the experience, even though it was a lot for for me to visually manage. The music is catchy too!