Longtime Archer C7 V2 here. OpenWrt 18 series works well with this router, but you need to consider that OpenWrt doesn't support the hardware switch on this router, so you are limited by the CPU of this router. Sharing with 5 or 6 users is just fine, but more than that, you will probably need to reset it every few days (or every day if you're using OpenWrt 19 series.). QOS can handle 50 Mbps just fine (Mine handled up to about 300Mbps with no probems. Anything beyond that will be beyond the CPU power of this router and you will experience problems)
I never got this router work consistently with the 19 series: Random disconnections, random freezes , random Wireless problems, dropped packets... In the end i just reverted to original firmware and i'm using a RPI4 to handle QOS, print server and a pendrive as network drive.
In your case, i think the best is to keep using the 18 series. Still stable and quite usable at your internet speed.