The main drive behind this endeavour has been Apologist's recent series of blog posts about creating your own army. It's something I've been wanting to do for a while and, coupled with a few other reasons, I'd like 2019 to be the year when I finally complete an article detailing my own Chapter of Adpetus Astartes.
The drive behind these concepts are, quite simply, just an idea I had last night. We see plenty of Space Marine Chapters rushing about the Imperium, which makes sense given their limited resources and mandate, but I am curious to explore the idea of a hybrid "standing force" of sorts, where a portion of the Chapter has garrison duties on the more important planets while the rest of the Chapter sails the stars to reinforce garrisons, ensure personnel rotations and deployment of freshly-issued or repaired equipment, ensure the continued loyalty of garrisons, etc.
With regards to the Adeptus Mechanicus, I imagine that they are on good terms with them. In many ways they are the only lifeline that the Chapter has, so they would go to great lengths to ensure the well-being of their relationship. Seconding Space Marines and ships to escort Mechanicus expeditions beyond the galaxy edge would definitely something the Chapter leadership would be ready and willing to do. However, given the nature of both parties, I'd say their relationship has had its strained moments. To avoid running dry during such times, they have probably made sure that their forges are capable of covering a range of basic wargear needs and they no doubt use their Chapter Keeps/garrisons as equipment and ammunition stockpiles.
With regards to the wider Imperium, I'd imagine they keep interactions limited to affairs pertaining to the defense of their protectorate. This might include stepping in to deal with any intercine Imperial conflicts, which would then beg the question: would they generally side with the Adeptus Mechanicus to ensure a good working relationship with them? If they don't get involved personally, perhaps they get on well with the local Inquisition who use the threat of Astartes intervention to keep Planetary Governors, Administratum officials, Ecclesiarchy ministers, and local military officers in line? Definitely something to explore further.
Concerning the Astronomican being weak, the Chapter no doubt sees itself as the physical manifestation of the Emperor's light to some degree. Where His psychic might is weaker, His luminous angels stalk the darkness and purge the shadows.
Using the Nova Terra Interregnum and/or Age of Apostasy to explain their isolation and distance is a good idea. Perhaps they got too involved in the intercine conflict and suffered greatly because of it? Could tie in to the Chapter not interfering in local matters mentioned above.
The downside of the Chapter is not being able to engage in protracted land-based (siege, armoured, amphibious, etc) warfare due to their resources being used to maintain their considerable fleet. As a result of this, their armoury would contain very few transports and heavy support vehicles (Rhino-chassis vehicles, Land Raider variants, Bikes, etc) and they would be forced to neutralise/obliterate their foes in void-based combat or within the first few hours/days of planetfall using Drop Pod assault and orbital bombardments. Beyond that they would have issues moving their assets around and would have to continually ensure air and void superiority to have a chance at reaching their objectives.
Perhaps this drawback could give me the chance to develop either a second Chapter that complements this one or their relationship with the local Imperial Guard who could help them fill that hole in their operational capabilites.
I would like to get them to the post-Gathering Storm/Primaris stage. I've started to warm to the idea of Primaris and would like to use this Chapter as an opportunity to explore them in more detail.
More stuff above and further more stuff soon.