Hello. I haven't read through the thread, but I have one suggestion if you're creating a fan-faction for 40k.
Every faction has some mcguffin that lets them have FTL, and in many cases it's one of the core features that defines the way of life of the faction. One of the core features of the 40k universe is that what ammounts to one of the only ways to go FTL is sentient and evil, and impossible to navigate without a mcguffin. Every faction needs one, and so should this one, whatever it is.
Imperials have astronomicon
Chaos worshipers make friends with and pray to the warp denizens and hope against hope that it works (and for them it often does)
Mekboys can build much more advanced warp-drives than other factions, because the old-ones literally hardwired their technology into their heads, but they have no idea how it actually works. When the Kroot ate Orks, this McGuffin spread to them as well.
Orks that don't have access to a sufficiently awesome bigmek can use looted warp drives and rely on WAAAAGH energy and WAAAGH plot-shield to protect/guide them to habitable planets where there's a fight.
The tau use warp-skimmers that don't go nearly deep enough in the warp to be logistically sound in most of the universe, but were lucky enough to rise in a cluster of stars that are REALLY close together... lucky them.l
The Eldar have the webway
Newcrons also have the webway
Oldcrons had non-warp-based FTL that they weren't sharing with anybody... their ships literally just went that fast.
Nids.... IDK, I heard something about space Narwhals.... I have my own headcanon about the shadow-in-the-warp that I personally think is cooler, but I think the current official stance is.... Space Narwhals
What is the FTL mcguffin of THIS faction?