How bout just adding a 'starbase' flag to tick that requires the design have no engines? Design it, build it. Except you don't 'build' it. You build sections, then ship them to the site, then assemble them. Much like a PDC.
Maintenance is easy. With the starbase flag you run a calculation to figure out how many minerals per x amount of time you need to keep things up. If the minerals are not present (as in, not in a cargo hold on the starbase, could do pull from docked ships but then you run into the problem of what if you don't want it pulling from ship x, but from ship y..easier to pull from built in cargo hold, KISS) then maintenance clock time is added. Now, maintenance supplies (as in the ship ones) should ALSO be required, again matching the mineral needs time. Should there be many needed? No...same with minerals. Now, keeping a stockpile of supplies on hand would be good, due to the fact that some items (just like ships) may break. It happens. No matter how well you take care of something, sometimes they just go splat.
For a good example, look at an aircraft carrier. They carry repair and FABRICATION supplies that can last their whole deployment time. Starbases should work just like that.
A self contained environment that needs outside supplies to exist. You could add hydroponics, green spaces, algae vats, whatever under 'Life Support' generic item. Make them need a stock of food. Let them count as destinations for trade vessels.
Should any maintenance module be needed? Yes I say. Make it starbase specific. Need x starbase maintenance modules per starbase section when fully assembled. There you go, the machine shops, repair bays, union meeting rooms, all the stuff needed to maintain a large structure assembled under one room
Should orbital habitats and starbases co-exist. Yes I say. When I picture orbital habitat, I picture something like...hrm..'the moon is a harsh mistress' or some such book. Where they exist to be a lay-over point for resting between shifts. That's it. Nothing fancy. Which means an ease of maintenance. Like the pile of overpriced crap we have in orbit, except you know, built well so it doesn't keep falling apart
Starbase I picture as..well..not a bernal sphere, to small scale. Nor a stanford torus for the same reason, though I guess they could be scaled upwards. I picture a o'neill cylinder to be honest as a starbase. I did even before the first time I saw babylon 5 way back when on tv (you know, when it first came out...I feel old..) and did a giddy dance about the o'neill cylinder...
Now, the advantages of doing them as a slightly altered PDC are many. The prime one we really really care about? It would make it easy for our man Steve to make a few changes to the PDC code and throw out starbases rather easily.
As for civilian vs. military starbases? I say allow up to x tonnage of weapons on a civilian starbase, when that point is passed, it counts as a military station. This would allow military starbases that can actually defend themselves. The military ones should of course require more maintenance modules per assembly unit. Along with a higher consumption of minerals and supplies. The manpower requirements just for maintenance and such should be lessened in my mind on a military starbase due to the inherent overbuilding that is all part of military design.
Just some ideas I had on the spur of the moment and spent a minute organising in my brain before typing them out...cheers!