A story about an Anthro PC completing its companion contract, Mary Poppins/Cat-in-the-Hat style, could be an interesting yarn.
The three companion APCs in the regular cast are Pintsize, Winslow, and Momo.
But of the three, the one that makes the most sense for such a story is Momo, not Pintsize.
Pintsize has an origin in early 00s webcomics, when every strip had a cute talking psycho critter sidekick. That may no longer be culturally relevant, but it's no reason to write him out of the strip entirely. He also had that one comic (can't look up # right now) where he flat out said (to Momo I think) that he'd done his bit for Marten and was content to sit back and proudly watch Marten spread his wings.
Winslow hasn't shown as much personality as the other two, and Hannelore seems the most in need of companionship and private emotional support.
Whereas Momo and Marigold have both developed as characters since we met them. Momo started as gag, then became a mascot character, and is now a whole person with roomates and friends and jobs, and also spokesperson for robot-centric stories. Marigold's also grown a lot, developed confidence and common sense, she doesn't need constant looking after, and she's got Dale too. So they seem already set up for a growing apart, but in a mature way, not like Harry and the Hendersons.