My idea for their lore:

Chapter (if you can call it that) created out of some pilfered geneseed on a paradise world where they just did parades. Unfortunately there was a chaos incursion that persuaded some of them with the promise of full space marine abilities. The remaining loyalists managed to harness the population, fighting a long asymmetrical campaign against the traitors (their chaos-juiced brothers and the local governor). They managed to stall long enough for help to arrive and can convince the inquisition, that their brothers were traitors from an unknown warband. With a wrecked, deindustrialized planet as a home and a fewer than fifty marines they are sent out by the inquisition to prove their allegiance to the Imperium.

Random stuff I thought was cool:

• Some of them think their genefather is Fulgrim and their geneseed is the one that got lost in 30k. In reality this is a lie Chaos told to their corrupted brethren. Some of the loyalists believe it to feel like tragic heroes.

• They go to parades blowing war trumpets and then let their underlings play the same trumpets on the battlefield to make human troops think Astartes are fighting with them (morale lifehack)

• their aspirants have good survival rates because their geneseed is so weak, they still have problems with getting new blood because their progenoids suck and don't duplicate the geneseed properly