The only one of them that was really abnormally gifted was Nyelbert and that's because he was literally bred and trained from birth to be a mage. Renda-Rae was just a hunter with particularly keen hearing, Brendan was a newly appointed knight who happened to catch the princess' eye and become her bodyguard, Lamitt was a novice healer searching for a cure for her people, and Ardbert was just a young warrior. Cylva may have nudged things here and there but I really don't think she can be pegged as having caused any of them to get their crystals in any meaningful regard.
My point regarding FATEs is that even outside of big boss encounters, basic FATEs found in the starting zones are at least as dangerous as what WoL did to get the first crystal, probably moreso. As for the Primals, again, they are really not very dangerous outside of the threat of tempering, Arenvald has been going around dealing with them in Eorzea since at least Heavensward and Stormblood and he's never gotten a crystal. Not to mention, for some reason WoL hasn't gotten any more crystals outside of the original six.Also you bring up FATEs like that matters. As if they aren't, from a story stand point, something the WoL does mostly by themself as well. The primals only go on to become more powerful, BECAUSE the WoL bested them. The Ascians stepped their game up, giving the Beastmen the secrets of sacrificing an individual to be the primal's core. The WoL doesn't canonically defeat more powerful foes until after the events of the MSQ, and in the few cases that they do, those certain FATE monsters that you are thinking of are in close proximity to that timeframe.
Frankly I think it's obvious the crystals are a dropped plot point from an earlier version of the story. The fire, earth, and wind crystals all came directly from Ifrit, Titan, and Garuda, the latter even being produced directly from her body and the former two seemingly having the same origins. Meanwhile Leviathan, Ramuh, and Shiva weren't implemented in time for 2.0, so the water, lightning, and ice crystals were all just obtained either randomly (for the one found in the starting area) or in some conditions that were vaguely associated with the element and Primal they were supposed to have come from. This is probably a vestige of the story from when the Primals were intended to be actual incarnations of elemental power rather than merely aetherial constructs based on myth and legend.
How exactly does this make any sense? I know early-game WoL is basically just a nodding automaton but there is absolutely no reason why they shouldn't need a test of character but somehow Ardbert and his party did, they occupy the exact same niche of "good guys". Hell if we consider the range of dialog options often available to WoL plus the sorts of things they've done in various quests, WoL's character is way more questionable than theirs was.Anyway, my point is that the WoL's character never needed to be tested. Hydaelyn knew from first Echo that we'd be the one to carry out her will and bear the crystals. Our power is ever tested, because we are immutably good.