Multiple explosions that cover up are the way to go IMO, they always feel quite good.
I think the enemy hitboxes being so small is the problem, you wanna separate the enemy body hurtboxes and hitboxes, making the hitboxes small while keeping the hurtboxes quite large, this is the best of both worlds.
Enemy wave design is tough yeah especially on a time limit since you don't exactly have the time to do a shit ton of playtesting to make sure it flows great, but well that toaplan left to right to left pattern never fails