Little update after running OpenWRT for about 24 hours. Overall stability and configuration has been wonderful! Although I find LuCI sometimes a little overwhelming in features. A search function would be a pleasant addition.
I'm however a little disappointed by the performance of the devices. When running iperf3 the speed caps at about 300mbps because of CPU throttling. I know the device is limited and CPU bound and I was looking into: Overclocking Router Devices . Also doing file transfers (over wifi) don't have constant transfer speeds, but rising and falling constantly. Together with the CPU utilization btw.
I read that someone overclocked his Archer C6 v2 with the same chipset to about 1GHz which resolved most performance issues. However I also found that hardware offloading can be available and there is a GitHub repo which has that.
Anyway, food for thought 