Clearly I must be playing on a vastly higher level of player skill if I managed to get 3 Atmas in one day... all of which by doing one FATE at a time and moving onto the next one after the current one finishes.

There's no diminishing returns or pseudo RNG from what I've experienced here; a 3% drop is a 3% drop, nothing else to say about that. I don't see how "rolling 3 dices at the same time" is any different from "rolling 3 dices after each other", considering you only need one of them to hit that 3%

I also feel that the concept of Atma just offhands the design of the game; you're not really helping lowbies or doing well in FATEs if you're swarming them with a party or simply tagging it and running off.

I can't help but see the same problem being repeated in the future; There's always an alternative which is both more accessible and more-or-less the same stats (ST and hunts giving tomestones and allied seals for unweathered weapons)