I'm interested in the same thing (both query upstream DNS over HTTPS servers, and answer to DoH queries), but using dnsmasq. I haven't had much time to look into this yet, but will definitely try to implement it later, so any recommendations would be appreciated. So far my research notes on this topic only contain:
Do you want Unbound to query upstream DNS over HTTPS servers or do you want unbound to answer to DoH queries?
For the former, unbound cannot query upstream DoH servers, only DoT and DNS at the moment. An issue is still open for setting DoH as upstream. A solution is to use cloudflared or dnsproxy to proxy DNS queries to DoH upstream servers.
Are you using the Unbound built into OPNsense, or something else? I ask because it's easy to configure Unbound in OPNsense for DoT. If your ISP isn't blocking DoT it will be just as secure.
And yes, it will be much more private. Right now if you're using neither DoT or DoH your ISP will be able to see all your DNS requests in the clear. With either of the above it will be encrypted and they will not be able to read them.