The discussion in the comments was useful.
That's very helpful.

My minisplit will be installed the end of the week. Going with a single head with some "wall fans" going into and out of the side room. I want to have a filter box, or like lazyman place a filter above, for the minisplit intake. Appreciate everyone sharing their experience. Sounds like a MERV 8 would be ok (higher may impact airflow based on this video), and ideally 4 inches thick.