The build is looking good so far. I would definitely recommend adding a second fan at the rear as exhaust.
I recently completed my M1 v6 build. Just waiting on some custom cables for my PSU and a power button cable to match my red and black scheme.
Here are my specs:
Ryzen 3600 @ 4.2ghz...
From what I recall, I don't think you need the back panel, but you won't be able to cover the front panel (like USB, power button etc.) without it. They will be just exposed.