Forgot to mention that all devices on this new 15amp branch will be 15amp devices (switches, receptacles, light fixtures, etc.) with NO 20amp devices.

Don't do it. The presence of the #12 at the panel would lead anyone to believe that the circuit could be properly protected at 20 amps even if you should change the breaker to a 15 amp. Use #12 for any extensions of the circuit.
You have no control over that property after your ownership ends-- and it most certainly will do that.

I would replace the 20 amp breaker with a 15 amp.

Just wire it with 12awg. The cost difference is trivial and you will be code complaint and safe going forward no matter what you or a future owner does. You can use 15A switches and outlets on a 20A circuit.

Go with #12, there is no reason not to

Thanks to all those that replied to my question. All replies were helpful.