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Here's what I have so far for the fluff of my chapter. They're sort of a dark take on reasonable marines.
I'm using 'rad dudes' as a placeholder for a chapter name I don't have. The rad dudes might be part of the Black Templars, or successors of another chapter. I haven't figured that out yet.

The Rad Dudes had a heavy cruiser called Fist of Glory that hosted 2-3 companies of their chapter, as well as thousands of chapter serfs somewhere in ultima segmentum, not too far from the ghoul stars. The Fist of Glory got a tip from a rogue trader of salvageable tech on an uninhabited planet called Murimac. When they went there, they found that an ancient battleship had crashed there, and was fairly intact. When they came closer to investigate, they apparently triggered some automated defense systems, and they crashed on Murimac next to this ancient ship.

Stranded on Murimac, they explored the battleship, called Alnei Heifa, for salvageable parts. They lost many of their librarians and chaplains in a power overload accident during the first few months, but they found a ton of archaeotech, and a main computer that was almost fully functional. Many STCs were present, but most were password protected. The password hints sent the marines searching through the cultural library of the ship, where they found ancient classics of literature and political philosophy. Over hundreds of years, the Rad Dudes made progress on repairing both ships and unlocked dozens of STCs, but all the while, their loyalty to the imperium was being called into question by the books they and their generations of chapter serfs were reading in order to find the passwords to more STCs. Eventually, they got all the information they needed to fix the Alnei Heifa, and then everything they needed to fix the Fist of Glory.

300 years later, the Rad Dudes finally fixed both ships and emerged from Murimac, but very changed. They've seen generations of chapter serfs change, lived hundreds of years with time for reflection away from constant practice for war, know just how horrible the imperium is and would like to do something about it, but if they attract the inquisition's attention, they could find themselves excommunicated. They've started gathering for themselves a power base on the edge of the ghoul stars. The Rad Dudes have been corrupted by the humanistic culture of the age of technology, but this makes them no less a threat to the imperium than a chaos cult.

Unknown to the Rad Dudes, Alnei Heifa has an AI from the dark age of technology. The AI learned about the current state of the imperium from the conversations or the Rad Dudes as they walked through its halls, and deliberately set up the power surge which killed the most fanatical, straight-laced and insightful members of the chapter. The AI remembers much of the war against the men of iron, and surmises that its side lost the war. Its motives now include at the very least survival, and that means keeping itself far from the mechanicum, which would realize what it is pretty quickly. The AI is considering revealing its existence to the captain who commands it now. In keeping with the grimdark and mysterious setting, it isn't clear (maybe even to the AI) whether the AI is genuinely benevolent, or whether it's just using humanitarianism as a tool to keep it safe and accumulate influence.

Should the 'rad dudes' be a rogue part of the Black Templars? Or their own separate chapter? If the latter, any name ideas?
Is there anything about the Alnei Heifa that I got canonically wrong?

On the table top, the rad dudes use white scars chapter tactics, and work closely with imperial guard elements and sometimes 'inquisitors' with cool (archaeotech) technology. Usually (but not always), I don't field psykers.

I field a competitive army and play competitively, but I don't want to neglect my army's fluff!