I've enabled Unbound logs and - with little surprise as Unbound is configured in order to listen only for queries from the local Pi-hole installation (on port 5335) - I've indeed noticed that all queries come to Unbound from IP 127.0.0.1 which is the IP of the local server hosting both PiHole and Unbound.
The issue is that, if Unbound cannot tell apart different IPs, I have no way of configuring DNS views within Ubound.
Is there a way/specific configuration for Unbound to "distinguish" the actual IP address that is making the query? E.g. the IP address provided from the DHCP server and assigned to the device making the request, which is known to PiHole but that gets then translated to 127.0.0.1 when using Unbound as DNS resolver.
Maybe, if unbound would support to parse EDNS0 ECS and consider requestor's IP information from that (preferably without exposing ECS upstream), but that would be a question for unbound support.
It would probably be easier to reorder your DNS resolution chain for Wireguard, putting in unbound first to handle requests from your Wireguard IP range by forwarding it to Pi-hole, with the exception of special domains you want to treat differently, as the music server domain you mentioned in your other post.
Also, I'm not sure if omitting all the context of your other post does help to solve your actual issue, which seems restricted exclusively to clients connecting via Wireguard.
Did you try my suggestion tackling your Wireguard issue by allowing access to internal devices yet?
Creating those PostUp/PostDown rules and enabling IP forwarding seems so much simpler, and it would also seem like the most natural approach, given that you are already connecting via Wireguard and data is served via your upstream connection anyway.