Do you have any idea how many of those I can spam? Even 4 star stuff I can all spam them without limit.

Terrible idea.

RMT has 2 great ways to generate gil:

Farm dungeon mobs (that for some reason still drop gil, square enix stop being bad and just remove gil from killing monsters, keep the gil rewards to the challenge log and duty finder bonus)
Farm dungeons for loot - all the craft mats they ever wanted, all the tomestones they ever wanted, etc.

And the "farming" method that we all know - just gather 24/7 and sell stuff on the mb