Cruelty and precision are pretty much essential to any mastery setup and should be your priority, though you can get md for free (well, energy) if you constantly run the mystic daily to farm mystic cores, and iirc you can get them with loyalty which doesn't really have a better use either so getting MD doesn't really interfere with that, assuming you have the mastery points (which shouldn't be a problem if you have 5*s and don't have the basic setup yet).
Cruelty and precision are pretty much essential to any mastery setup and should be your priority, though you can get md for free (well, energy) if you constantly run the mystic daily to farm mystic cores, and iirc you can get them with loyalty which doesn't really have a better use either so getting MD doesn't really interfere with that, assuming you have the mastery points (which shouldn't be a problem if you have 5*s and don't have the basic setup yet). The mystic daily quest gives cores?