Your lan hosts are querying OpenWrt DNS server for name resolving. You should use dhcp option 6 to advertise them another nameserver. The way you have configured your router doesn't guarantee which upstream DNS will be used.
After playing with various arrangements I settled on NOT passing DNS requests over WireGuard since it lead to weird issues. Instead I set up DNS hijacking and stubby and rely on encrypted requests sent out that way. All of that can be set in LuCi.
By default traffic is hijacked by router and sent out via DNS over TLS to CleanBrowsing Family Filter and otherwise televisions/iPad are sent to Clourflare to access otherwise restricted content.
So in my case I put up with television content related traffic going over plain text. The alternative for me would be two stubby instances or forwarding cloudflare over WireGuard, but again I think that gives weird effects.
If the wireguard server is also running nameserver, then you could. And it would direct all the queries from clients to that nameserver. Isn't it what you are trying to achieve and you say that there is DNS leak?
The issue is I don't think the VPN server that I am using allows port 53 unless you are connected via the VPN. I have tried to unblock it without success. It could be the VPS provider blocking it possibly.