Regarding your suggestion of an extra team to handle collabs, they can't just pull developers out of thin. Yoshi-P has talked multiple times about the difficulty of taking on new devs able to to work to the standard they need, despite wanting a bigger team. If they got more devs, they would want them working on the main game so a separate collab team would still take away from development.

If it was made of a less skilled team if "learner devs" the resulting collabs would be of poorer quality than the rest of the game which would leave a bitter taste for players and could be considered insulting to the properties being collaborated with. Would likely result in less collabs as who wants their product to be represented by the worst content in the game.

You keep bringing the Nier raids, but these have actually brought up a lot of push back against more collabs in online spaces. Much as I like Nier and the glams and minions from these raids I found the raids to grow boring quickly and the story quickly and in hindsight would have preferred an alliance raid series that dug further into Norvrandt's lore, especially as we're probably not going to get much of a focus on that going forward.

Ultimately, it seems the devs have taken note of players wanting fewer collabs and more FFXIV lore-based content as in an interview with Frosty Mogborn earlier this year Yoshi-P said they are not currently moving forward on any collabs and when revealing the 6.x raid series they made a big thing about it being XIV original content.