Simple:

Options, even if you'd lock the relic behind just OC itself, you should have options of doing other things in it than just FATE's for the target.
Well what else is there other than Mobs and Fates in OC that's similar busywork? Well if there is non, how about you add some basic exploratory stuff in there, like I dunno;
Collecting wilds animals or something.

It doesn't actually matter if it's tomestones, lightgrind, atma or whatever, as long as there is multiple ways of attaining those things that doesn't;

1. result in repetition
2. become too easy

To achieve this you kind of have to allow each step to utilize alot of the games regular content. FATE'S, Hunt Trains, Duty Roulette, World bosses, Deep Dungeons, Variant even Raiding & treasure maps are allowed. Simply becuase things to do inside OC is very limited (but you could add things in there as I said)
Also instance based content like doing Eureka and Bozja would have to have means for you to attain something to the relic step, so that people who have kept those in the backlog have a CURRENT goal they can bring into it.

The hardest part is balancing how much you gain towards the step from doing certain tasks versus others and thats related to difficulty of the task.
And people would ultimately find the 1 method that is best.

The bad part is that when people find the 1 method, SE barely ever implements a simple patch that increases or decreases the ammount
and if they do, it's too late. Other MMO's do adjustments on almost weekly basis. But for SE a simple value change needs a meeting & 3months of development time.