Mastercrafting. Have raids and other serious content (to be added, I guess) drop actual parts. Some are the chassis, while others can only augment or act as base material for other chassis.

You can build up rep, essentially, with different NPC artisans and so forth so they can augment, install, or assemble these parts. Or you can find player crafters. Or be one.

You build your own relic.

Granular, but with noticeable breakpoints to be hyped about. Few if any parts are RNG-based. Difficult content may be required, however. Grindy options allow you access to easier combat tasks by which to instead acquire the given part, but everything of value should as if directly from something difficult for the individual pursuer.