Have the system in place where everyone can cap within the same amount of time, and adjust as you go based on data received.
I'll agree that SE isn't likely to come up with an elaborate system of tracking or even just monitoring progress, but if everyone can end up on the same level fairly easily then why bother? Everyone would end up on the same or a close enough level of performance that they all deserve the same chance at loot so why would this be different from just saying "Everyone has x% chance of getting loot if they complete the event"? It's basically the same as just having a flat drop rate for everyone who participates in the event.
I think it depends on what level SE decides to put the content. You mention that if everyone can end up on the same level "fairly easily" they might as well give everyone the same rewards just for participating which I think is true in the sense that it might be the better option just because it means less developing and attention from SE so they can focus on other things. It is the easy way out after all.
However if the level of effort and skill required is quite high to reach the highest rewards I think we get a different story and if the rewards are going to be items that are actually pretty good I definitely think a system which differentiates based on effort is the superior option. It would require a lot more effort from SE in balancing so that all jobs get as close to each other as possible when it comes to effort vs reward though.
A system like what Kachi suggested makes sense to me tbh, each job has different objectives entirely so that everyone gets a chance to reach the higher tiers entirely based on effort and how well you can play that job instead of what job you are playing.