alriiiiight, I have 1 main and already leveled 3 other characters on different acounts to level 18 where you can start doing elder rifts to farm FA rune. For FA rune I craft random 1 star gem to sell it for platinum and then smuggle platinum to main character.
The thing is, you farm FA rune with your main in elder rifts anyway, so if I have 3 afk alts there, no problem. I will see if I can make 300 platinum daily easily as well.
This is more for fun and experiment, I think it will boost the platinum income pretty well.
I will probably have 1 main and 3 + 3 alts, so you can farm with first 3 alt batch and then with second 3 alt batch, as max party is 4 in this game.