What makes a good relic step? Having it be a bit of a grind, so it feels rewarding to do, but not too big a grind so as to make alts hard to work on.

The solution would be something akin to the Blade's X step from SHB. One grindy 'onetime step' to unlock the vendor/process/whatever, then all the alts thereafter are a simple small grind. In Zadnor it was 'get 30 of these 6 different kinds of items, either via Zadnor stuff or old raids', and the 'easy repeatable' part was 'do Delubrum for 2 memories, Dalriada for 3, relic asks for 15'.

For the Manderville relics, since it's all about bloody aliens and space and meteorites, they could have had a section of the Thavnair map dedicated to it where FATEs spawn, meteorites keep landing in a starshower and you have to pick up the fragments and hand them in to an NPC, and you get given a share of them once the FATE ends to use for your relic. That's the 'hard, onetime step'. Then, the repeatable part is the 'buy this garbo for 1500 tomes' (which could even be lowered due to the onetime step being part of the 'grind'), make it protein powder to coat your relic in for all I care. Make the first one free as part of the questline too