Hi OP,
My suggestion, which you’ll have to double check is feasible, is to keep your moon and upgrade to the MRX 720.
I believe (and you need to double check) that all 7 channels are re-assignable. Which may mean you can use them all for the non main (L/R) channels. Looking at section 5.1 of the MRX 720 manual, under "Amp Matrixing" this should be possible.
https://manuals.anthemav.com/mrx-avm/en/103
Best,
Erik