After reclassifying to the 15T PMOS, should I go back to 12B?

If you're planning to wait till you promote as a 15T you're going to be waiting a lot longer than you would as a 12B.

In order to switch to an MOS which used to be your Primary MOS you need to contact the MOS Proponent of your old MOS and request a memo stating you're still qualified in your old MOS. The longer you're out of your old MOS the more likely they are to say no, or require a skills validation test, depending on the MOS.

Then there's the Army Strengths portion of the reclass request. Your old and new MOS are both balanced so you can't reclass outside you're window without an ETP and there's nothing exceptional about you or your case so far, so HRC will deny it. You can do it in your reenlistment window, possibly. But that's probably another year or two for you because the SRR for 15T is three years from graduation of AIT. If you're a Careerist now, meaning your ETS is at the mark of ten years past your BASD, then that will only be an option for you if your new MOS receives a Tier 3 bonus at the time of your request, which it currently does not. It almost never has a bonus for SSG.