ASUS RT-AC86U with Koolshare firmware (derived from Merlin firmware) has Wireguard binaries, but no GUI configuration for it.
I expect that any Asus router that can run Merlin should allow you to ssh in to install Wireguard. But you might need to cross-compile it on another system, and obviously the lower end models might struggle with CPU usage.
The Koolshare group (whose modified version of Asus Merlin is targeted at folks in China) have stopped development for the RT-AC68U, and IIRC this was before they started working on integrating Wireguard into their builds.
It looks like Asuswrt-Merlin uses different kernel version for different routers (probably because the use the kernel from Asus' own open source releases). I'm pretty sure the kernel for the AC68U is too old to support Wireguard.
I expect that any Asus router that can run Merlin should allow you to ssh in to install Wireguard. But you might need to cross-compile it on another system, and obviously the lower end models might struggle with CPU usage.