If you're Mithra or Hume, you don't really need to work on MP that much, either—you chose Elvaan, though, so, yeah, you will need to get +MP gear if you want to be a mage.

Subbing BRD works in very rare cases. I know a very nice girl that is a DRG/BRD, and we are at high levels (60+). She subbed BRD because she was sick of missing with her DRG, and she can cast a song on herself to increase her accuracy. She also helps the mages out just a little bit with her ballad.

In the end, what you sub as a White Mage depends on what you want and what would benefit your party as a whole. Nobody is going to NOT invite you at higher levels because you subbed RDM instead of BLM or something like that, because finding healers or support is very difficult, but would you rather have limited spellcasting abilities to attack the enemy with (Black Mage) or be able to heal and further support your party, as you're already doing, with White Mage?

And by limited, I mean very limited. Anything that deals damaged skillwise, subbed, will do next to no damage at all. You cannot expect to cast Fire II as a White Mage and do any damage at all.