Unbound can be run on the same device as the Pi-hole, you would just need to run Unbound on a different port and then change the upstream to point to that new port scheme.
I run multiple Unbound installs on different VPSes and then forward my Pi-hole to those Unbound installs as upstreams.
We have been looking for a while at how to allow users to change the resolver used, and to see if it's feasible to be more modular in the installation. That will take a lot of time to work out, but it's something that has been discussed by the development team.