If you do go the 9mm route, these Foxtrot Mike lower seems to run well. Not as sophisticated as the CMMG radial delayed blowback, but they work and are almost half as much.
Here is complete lower package:
https://aimsurplus.com/foxtrot-mike...-19650353&mc_cid=a2f765fd06&mc_eid=f0d434d8bd
[EDIT] Wow! "Aim inside". I think Intel is going to sue these people.
My blowback 9mm upper is a FM. It runs great I just wish I hadn't gotten the 3 lug barrel. I though it was a muzzle device but it's machined into the barrel. Now I need a 3 lug suppressor for it...