Let's just use mage as the generic "occult magic user' tag. This is what a lot of English-speaking people would do in the context of fantasy media anyway. That's how DnD uses it. And a lot of games. [Don't want to (or can't) exactly describe someone as a "wizard", "wizard", "warlock", "sorcerer" "sorceress", etc. etc., just use mage. I suppose "magic user" would also be okay, but that's more syllables (and letters), and spellcaster might cause some issues because some people only consider "spells" to be a specific 'kind' of magic, i.e. something that is formulaic and/or systematic.
So if one wants to find a specific gendered "magic user", then mage + the usual 'gender' tags will work somewhat adequately.
Then let all the various different "occult magic user" tags (Wizard, Witch, etc. Not Priest and the like though, or "divine magic user" in DnD terms) imply mage. Doesn't matter if it's a generic cosplay, if it looks like a "magic user garb", then it gets implicated to mage, easy as that (Yes, not like how I suggested earlier, but that distinction was kinda silly and makes things harder to judge). Now all the mage tags on Mona posts fit.
Leave most of 'mess' with Witch and Wizard and Sorceress alone. If one finds anything that is 'mistagged', that is, fits better with one tag than the other (e.g. Witch instead of Sorceress), then change the tags accordingly.
(And maybe also [re]write some wikis to help clarify the usual "stereotypical design appearances", especially for ESL users who are not familiar with how those words usually mean for English-speaking fans of fantasy-setting media.)
This keeps the status quo, which seem to be working fine for people looking for Witch and Wizard posts, make all the "job tagging" of Mona fit within this "general definition" of mage (though all her posts should also probably be tagged Witch on top. And all her images in civvies or alternate costumes should have both removed). It also lets us use mage as a generic tag for "occult magic user", which seems to be how people was using it before Mona. So, overall, I'd say win-win for everybody.