Custom iso without local_unbound

It will be a lengthy process but ideally you will want to build your own release(7). It looks complicated but it's really not a lot more work than a basic buildworld. With your own release(7) you can set up src.conf(5) and set WITHOUT_UNBOUND.
The first part is the same process as for source upgrades/updates, it's the usual buildworld/buildkernel process. The release(7) man page is all you need, just look at the examples.