I think simply TM works as per the normal way the movies have been acronymed. Just M doesnt make sense to me as I’ve never discounted the “The” from TESB, TPM, TFA and TROS.
Some alternatives I can offer up though:
MNDO - Short for Mando and Mandalorian
MAN - Unmistakable as meaning Mandalorian
TITW - This Is The Way