Down thrust is used to counter the offset drag of a wing that is mounted well above the thrust line. Dihedral increases that offset.

If you are not going to remove dihedral & lower the wing, I suggest that you add a bit of negative incidence to the wing -- start with a 1/8" shim under the trailing edge & go from there. You will need to reduce the down thrust if you do that, & you may have to trim in a bit of up-elevator. Your inverted flight will improve noticeably. I have found that as much as 1/4" of neg incidence works reasonably well on most standard trainer types, without introducing serious negative effects.