If you’ve done many of the dungeons on normal, my advice would be just pick the one you enjoy the most. There isn’t any rigid hierarchy of difficulty beyond the dungeon, dungeon v2, DLC dungeon.
It might not be the easiest, but you’ll feel most comfortable dealing with the step up to vet in an environment you enjoy. Similarly if you can go with friends rather than pugging you’ll likely feel more relaxed.
It’s a bit of a step in all cases, you’ll run into new and more serious mechanics, but essentially it’ll be the same space, the same bosses with a bit more complexity. So if you can handle the AoE from the final boss in normal FG1, you’ll find the same mechanic, only this time it will almost certainly one-shot you if you get caught in it.
And don’t forget, it’s OK to die. Everyone here has done. Repeatedly.
Good luck and have fun.