I've tried both Openwrt and DD-WRT. To be honest, stock 18.104.22.168 and 22.214.171.124 are the most stable firmware so far! Especially if we want to enable 2.4GHz/5GHz as well as the corresponding Guest SSIDs all at the same time (openwrt and dd-wrt were unstable under such setup).
Just updated from 126.96.36.199 to 188.8.131.52. So far so good.