https://warhammer40k.fandom.com/wiki/Astral_Knights
Fluff wise, it is said that it's Brother Thade as the wise old Master, and 30 Brothers and Neophytes.
I assume that since these were they guys left behind, we would have a techmarine to keep the lights running, a couple of relic bearers/honor guards, some rearguard/wounded, a veteran instructor or two, and some wet-behind-the-ears scouts.
They are described as Imperial Fists Successors.
I was thinking if making them all first born, since this was before the Primaris.
I could use a mixture of MkVI
HH Marines / Cool loooking Firstborn models and Scouts.
I could grab any Dreadnought and kit him out to make him look the part of a Chapter Master. He doesn't need to be an actual special character.