What would be ideal IMO is a bug with a gut bacteria exclusive to that species alone that could eat plastics and digest them fully so microplastics aren’t an issue. Likely, a species for each type of problem plastic. A natural analogue would be termites, which can only digest wood because of such a relationship.
It would have to be an artificially engineered relationship, and an insect that’s not particularly proliferate. Preferably with a narrow set of habitat tolerances. That way they could be farmed, but be unlikely to get into the environment and become a nuisance by eating plastics we don’t want them to.