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.