Just to close the loop....

The ASUS RT-ac68 is a nice looking piece of kit, albeit it's a plastic case.

I flashed the latest Merlin firmware on for scripting and openVPN functionality, and after a couple of days tweaking have everything setup as per my old router. The merlin firmware looks a bit "neon glow" for my liking, but with a little coaxing and some custom scripts, is reasonably extensible. Ran into a couple of quirks such as a problem with dnsmasq rules and needing a full reboot after making setting changes instead of just relying on the router properly "applying settings" without a reboot.

LAN performance is slightly better over the ethernet-over-power network than with my WRT54, which is nice. As per predictions above, there is a full 2.4GHz signal around my house, but as the client devices can't transmit back to the router, they can't connect any better than the 'g' router. I haven't noticed much speed difference in other areas, but with fibre coming later this year, it's good to have the needed and available routing capacity in place....although in due course I may yet relegate the ASUS to a wireless/wired AP, with a pfsense box attaching to the fibre ONT.