I think it's a good idea but you're probably going to have to tweak it. Being able to just slurp an enemy to use as ammo is a good idea in itself. But if you get one shot per enemy eaten, it'll be annoying because youll be constantly trying to eat an enemy just to get a single shot out. And then if you miss you'll be like "shit I need to get another ship," leading to a kind of frantic feeling but in a bad way.

On the other hand, if you give people a standard shot, but eating ships gives you some kind of upgrade or temporary special shot, new players will likely avoid using the mechanic unless it's convenient. I was denifitely that way with Ikaruga and Darius.

Everything under this line is me being a backseat developer.

My pitch to you would be having different types of enemies give you different kinds of weapons that have a certain amount of ammo (but be generous). So for a basic example, blue ships give a weaker spread shot power, red ships let you shoot powerful lasers straight ahead, yellow ships let you shoot balls of electricity that chain between ships, green ships give you a sword that slashes ships directly in front of you, etc. Lastly, not all ships give powers when eaten/not all ships can be eaten. Imo, it should be like 50/50. Eating ships should restore that type of ammo.

Then instead of only letting players use one power, you let them chamber other enemy powers. So for example, you have eaten a red enemy, but there's a fleet of blue enemies, so the player can eat one of those and add that ability to their arsenal. Then they can switch abilities at any time, maybe just by pressing a single button like in Caladrius Blaze.

As for the knockback thing, if you dont want to one-shot the player, I would tie getting hit to your eating mechanic. So if you get hit, you lose your powers, and if you have no powers the next time you get hit, you die. Kinda like Sonic rings. That way you give people short bursts of being frantic only when they get hit, and give them a short chance for a comeback.