Thanks, appreciate it, I looked a bit into it, the DNS routing in OpenWRT is a black magic, ton of options to configure dnsmasq, then also the DHCP server what it gives out as DNS (there’s so called "option 6, when you can basically instruct DHCP in OpenWRT to hand out 3rd party servers, not its default router localhost), when it uses forwarded DNS, when you define a different upstream one, when it excludes e.g. “.lan” domains.
I think I’ll handle this OK after ton of experimentation, thanks for your offer to help!
What’s more interesting is that if you go to the router’s diagnostics via LuCI, you’ll see that the router itself can’t resolve addresses on the WireGuard network, despite the DNS settings being in Automatic and the “DNS from WireGuard” being the proper address. The router just won’t use it, at all, ever.
If you set your client’s DNS server manually to the WireGuard server, it works perfectly.