Ath9k is not going to win any performance awards anymore, but it is still pretty rock solid chipset and more importantly has afaik the very best foss drivers. Bit odd that they are basing their firmware on OpenWrt 15.05, which is over three years old now. I guess flashing in 18.xx shouldn't be that hard
The current development version of LibreMesh[1], the LibreRouter firmware, is based on OpenWRT 18.06.1.
The soon to come first batch of production LibreRouter units will come with this version pre-installed.