General Crystal RP discords are [nFAxTfd] and [jVXurfS].
The Balmung RP one is [bnBSTeU] and the Mateus RP ones are [XsBTbZv] and [mateusrphub] (yes, that's its vanity join code; not a recommended recommended server t. anon). Those three are the most active and with world visit they're not restricted to just Balmung/Mateus players or anything.
Far Eastern RP can be found at [EGtnH3] and [3Z7QSVz].
Garlean RP is at [vpeNZhT].

Hope that helps! Most of those places have a channel where there are more links to other discords which focus on specific sections (like Ishgard, Miqo'te tribal, First Shard, etc.)
There's also tinyurl.com/whereistherp which is a Google Sheet that has more links in it to FCs and other discords in an effort to actually organize all this in one place, though it's far from complete.
Finally, Fellowship Finder isn't entirely useless for finding links to RP discords either, even though Fellowships themselves aren't the best.
And just remove the brackets around the links, this thing got flagged for spam like 5 times. I hate 4chan sometimes.