One thing about 87!Donatello's occasional desire to be a normal turtle is, it's worrying, but it's not suicide adjacent the way it would be for any other Donatello.
The context in which he starts to think about it is one where they've pursued a newly mutated sumo gerbil (yes, seriously) back to the petshop where it was trying to break in so it could "go home". When the unstable mutation wears off it's happy to be shut back in its cage and Donatello thinks he recognises the petshop as the one where the turtles were originally sold.
For the gerbil being given sudden sapience isn't a gift or the beginning of its existence as a person, it's a weird and scary thing that left it adrift in a world it didn't understand and that didn't understand it. Having it wear off is a happy ending. After all, if a human was turned into a dragon and given massively expanded intelligence, a thousand year lifespan, and the power of flight and fire, but still wanted to go back to being human that would be understandable.
Donatello remembers being a turtle - his sense of his own identity doesn't start with sapience - so seeing another animal unmutate and be happy when he's unhappy with nearly but not quite fitting into the human world makes him thoughtful.
There's still a lot he does and enjoys that a turtle couldn't do! And most of the time (such as when being threatened with de-mutation by Shredder) he doesn't want to go back to being a helpless turtle. But there is that lingering feeling that he might fit into the world better (and be liked by humans better) if he was simply a turtle they could keep as a pet.
