I've tried both Openwrt and DD-WRT. To be honest, stock 126.96.36.199 and 188.8.131.52 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 184.108.40.206 to 220.127.116.11. So far so good.