Square-Enix have said before -- in these forums, even! -- that they don't care if folks are using ReShade or its forks (the FFXIV-focused GShade, or Nvidia's own Freestyle -- which actually lists FFXIV in the officially supported games with Square-Enix's blessing) to tweak the game's display for artistic screenshot purposes. They've just asked that those sort of screenshots don't put the normal copyright message at the bottom, so that people don't mistake them for stock shots.
They've also sort of indicated they don't really care much if folks are modifying poses/emotes/positions in GPose (when character data is entirely separated from anything the server cares about); if you've got a screenshot sitting on a railing reading a book, cool, enjoy. (Though if folks are using it to make porn they do sometimes step in and drop bans on them, likely because they have to if they want the game to maintain its existing age ratings.)
They actively care if people are using memory modification or network hackery to alter the game state in ways the server cares about, though. Botting, etc.
It sure would be nice if GPose included a lot of those options on its own, though, I grant you this. (Manually reposition your character, create custom poses, change the time-of-day and weather, etc.)
The reason game companies (and other places) write very broad TOS terminology ("If you use third-party tools you may be banned.") is so that they don't have to constantly revise it. If they said "No third party tools that alter the client's memory state in a manner that affects gameplay", someone'll make a tool that modifies the outgoing network traffic entirely separate from the game process, point to that and say "I didn't break TOS!" And worse, they'd be right. And then Square-Enix would have to sigh and go revise the TOS again to add something else you can't do, and wait for the next person to find a loophole.
Just saying "No third party tools or you may be banned." means if someone's using third-party tools in a manner you don't care about (i.e., shader injection for artistic screenshots, DPS parsing for personal improvement in your static, etc.), you're free to go "Meh. *shrug*" and ignore it, but if someone's using third-party tools in a manner that's problematic (i.e., botting or cheating), you're free to go "You broke the TOS, you are the weakest link, goodbye." and toss them out the door.
So, "What will stop players from doing other more cheaty things" is "if they get caught, they will get banned", as Square-Enix is free to enforce their TOS in that manner.