Gonna catch them all! Too bad there is no achievement for collecting all the parts, but I'm pretty sure you didn't intend it to be possible for players to collect all of them. But here I am again...
Here's how I did it (testing branch version): I got to the last boss, and it's awesome, and it has all the parts and rotates to attack me with various strategies, and there are also 4 supporting mini-bosses that come to help the boss when it's in trouble, and you only finish the level once all 5 of them are dead, but zooid part drops from the main boss once it's dead and if I quit out to menu once I get it before killing mini-bosses, I can continue the level from start, with all the bosses alive. Even though it's hard, one can farm last boss for parts and quit out as long as they don't kill mini-bosses, as the game ending sequence does not activate. This can make free roaming level quite fun :D