I am proposing drastically nerfing the mobile depot while offering somewhat of a replacement as well as something to fit in the lineup between the mobile depot and the Astrahus citadel.
Outposts, or Micro-citadels:
These would come in three sizes and would function similarly to citadels but would be much cheaper and easier to set up, much cheaper to run, but also much less powerful overall. They would have a docking capacity limit so in addition to having a low size limit for docking, you could only fit so many of that ship size inside one of these. The rest could be tethered outside, however these would also have tether size limits. Their defense would be weak.
This is the largest outpost size, and it allows docking up through battlecruiser size, and tethering up through battleship size. Unlike an Astrahus citadel, this structure is unable to fit station service modules. It has a full fitting and packaging service as well as inventory access, and it can repair ships that can dock or tether. This outpost has a built-in defense system which will automatically target and return fire upon anyone who fires at it. The defense system has the power of a few battleships.
This outpost could be carried whole inside a freighter and deployed into or retrieved from space directly. It would take a few hours to online, and its reinforce timer would work like the mobile depot and mobile tractor unit.
This is the mid-size outpost, which allows frigates and destroyers to dock, and allows cruisers to tether. Docked ships have full fitting and packaging access, but no repair service, however capacitor is regenerated for any ships docked or tethered to the outpost. It has a built-in defense system which will automatically return fire, and has power similar to a single battlecruiser.
This outpost could be carried whole inside a deep space transport or Orca. It would take several minutes to online, and its reinforce timer would work like the mobile depot and mobile tractor unit.
This is the smallest outpost. It has no docking capacity but enables ships to do in-space refitting which does not include subsystem swapping. It has no capacitor service and no packaging service. It also has no defenses. It has a decent amount of storage space along with being able to tether frigates and destroyers.
This outpost can be carried whole inside any regular industrial. It would take a few minutes to online and its reinforce timer would work like the mobile depot and mobile tractor unit.
Mobile Depot and Mobile Tractor Unit:
I feel it’s too strong to be able to have such a small and cheap fitting service. I suggest making it cost more resources to make, and take up a lot more cargohold space. 500m3 seems reasonable, as it can still be fit into battleships but suddenly takes up most of their excess space. I would also increase the size of the mobile tractor unit, perhaps to 250m3. That will leave battleships with plenty of extra space, but at least having the thing will substantially reduce the space you have. I know I’d still use an MTU even if it cost 250m3 cargohold space.