Good start. I always found adding the squad and chapter marks to the shoulders and knees is a relatively small step with a large bang for its buck. The tactical arrow is pretty straightforward and their sharing the fist chapter badge you can for sure find decals or a simple painting guide. You can always practice on your first born.
For your headcannon maybe add some bits and bobs in honor of the original third? Maybe black purity seals? I’ve never seen that. Some kind of knee badge? I don’t know.