Treat this like Food for thought: If the Imperium managed to reach a peaceful point to where many space marines aren’t really needed, but the Imperium does not wish to “retire” them like the thunder warriors… what kind of jobs do you think space marines could do for the Imperium outside of combat? Be it to entertain themselves or to actually feed and house themselves. What about the specialist space marines such as Chaplin’s, apothecaries, tech-marines?

a-40k-author:

This is a pretty fun thing to consider. I can see the specialists like chaplains and apothecaries finding ready jobs in hospitals/medicaes and the Imperial Cult. Some Chapters would also adapt more readily that others. The White Scars, for example, already pursue numerous arts and philosophies that could open up varied careers for them. Others, like the Iron Hands, would certainly struggle. Most would have a chance though – we can all imagine the Imperial Fist builder, the Ultramarine scholam teacher, the Salamander blacksmith, the Blood Angel artisan, etc. 

Another consideration: what would the joining process look like?