Don't worry, I completed the Omnilex Atma and I can tell you that 99% of the FATEs in this game can be entirely made in solo, especially by Summoners, Bards, White Mages and Warriors.
If there's no one in the FATE (which for me happened only with "Sharknado"), obviously you'll have to do it alone 0-100%, and this makes you unable to go to other FATEs, but if you stay, in Western la Noscea for example, between Aleport and Swiftperch, you'll get plenty of crowded FATEs at any time of the day and you'll be able to farm more efficiently.
The drop rate per FATE isn't that low for an RPG, if you remember FXII, the Tournesol sword required lots of ingredients much more difficult to get, like the Hellgate's Flames from Cerberus's in Feywood, which couldn't be stolen and just dropped with a 3-6% probability (6% with tier 4 killchain), or the Wargod's Bands from Leynirs, they were 3 in all the game and you had to move through 3 enormous zones to make them respawn, and they dropped at a 5% probability.
Atmas are definitely easier than a tournesol, which requires the time of an Animus at least to complete. And a Danjuro requires the time of a Novus (at least).
You just have to accept a game as it is, and know that if you want a grind weapon, you have to grind. If they made the drop rate higher, it wouldn't even be a grind or farm, you'd get a weapon in a day just FATEing casually. I'm gonna do another Animus after this, and I'm happy to grind again atmas, I like spending my time like that.
One thing I've never understood is why people play for the rewards. No one in this game is enjoying a grind or a dungeon, everyone just wants to get to the loot and go in another raid to get other loot: everyone should know that this is a game, and the purpose of a game is spending some time having fun and chilling out. If you're not having fun grinding atmas, don't grind them, SE can't make something easier just because people can't get the loot.
No offense huh, it's just a fact.