CB, I really can't see why you would need a separate amp for your setup. The Anthem may be an improvement over the Onk...maybe because of ARC. I can't say because I've never tried.
My 2012 Onk was a huge improvement over my now 18 year old Denon. I have to say though, v4 was an even bigger improvement over v2. When I put the Denon and the Onk side-by-side to drive M3, M5, M100, 160, QS10, 800 all v4, against M80, 600, 150, QS8, all v2, I'd take the Denon with v4 over the Onk with v2 any day! The v4, any v4, is simply excellent.
Your M5s are solid for your room size. Before spending on a new receiver, try QS10s and a sealed sub. Compared to the QS8, the 10s are a big difference.