if it was me i would build my own upper & lower saving some cash for ammo and re-loading stuff.
AR-15s are so easy to build piston and kits are not very expensive either. you could save a ton of $$ and get the important thing--ammo.
if money is not an object maybe a SCAR or an LMT qcb piston.
have you considered building a 6.5 grendel or 300 blackout? the 300bo is getting some traction as it is just an up-cased 5.56 making it cheaper than alternative up-sized rounds for the AR. uses the same mags and bolts
saving $$ and you can go back to 5.56 with just a barrel swap --
at least look at the 300 before you decide.