I do not think the mite population has yet been greatly impacted by the PAM. The tub is easy to clean, so I think the problem must be from mites on Nim's body. Nim is soaking a lot and seems to drown a number of mites each time he does, so I am considering moving him to the larger tub this week. This will allow me to give him a water dish large enough to fully soak in at the cost of more difficult enclosure cleaning. I'll manually bathe him immediately before the transfer, and hopefully that plus bathing dish plus ongoing PAM is enough to prevent the mite population from bouncing back.

Maybe this isn't necessary and the PAM treatments just need more time to work, but I don't think there's much of a downside to using the large tub. Maybe it costs a little more PAM. Up-sizing will also make feeding without getting rat blood on the carpet easier.