I’m really sorry - my experience with wireguard is limited! I’m sure that I tested to see whether it worked with adblock or not and found that it did though - I hope I am not leading you down the wrong path (this was fw 3.025 btw on ar750s).
Anyway, now that adblock isn’t slowing you down, can you try to select wireguard interface instead of ‘WAN’ in the luci adblock settings? I found that openvpn was listed here (and this helped greatly before I worked on making adblock run smoothly) but I don’t have wireguard installed in my current build to check if that would be an option. If not, maybe try another dns option such as unbound? I believe that glinet uses dnsmasq anyway but I don’t know what wireguard is doing wrt to DNS on top of the existing build.
Perhaps you could try a manual wireguard config instead of using the mullvad configuration tool in the gl.inet firmware?
Did you try overiding the DNS with the gl.inet custom DNS option?.. Perhaps try the DNS over TLS option to see if that works/gives any clues.
Honestly though, these suggestions are off the cuff and hopefully someone with under-the-hood wireguard knowledge can help further…
But at least you’re not waiting 10 minutes to load the adblock lists any more 