To some extent it probably depends on what level of "mobile" you're designing for. Like all starbases must have at least some small ion thrusters for station-keeping. You could probably expand those somewhat, and have something which still did not take up too much of your mass budget, but would let you move between orbits in a few days, or across a system in months. The real kicker is warp travel, because so much of the ship has to be designed around it, and so much of the mass budget goes for it.
Probably there is a niche for a ship which is halfway between an overgrown defence monitor and a station. Mass-efficient ion engines and the kind of slower warp drive you'd normally see on a commercial freighter, and then the most coked-out guns possible, and lots of plating and shielding. Not really that effective as a warship, but if you need to defend an orbital, or siege a defended orbital contested by an enemy station? Park six of them there.
A very different approach might be to have a modular warp transporter system which is not normally installed on the starbase, but can be used to redeploy them. Something halfway between those hyperspace rings you see the old Jedi starfighters using in the prequels, and those massive barges used to move entire oil rigs. Given that a ring-type is the most simple form of warp nacelle, and Federation starbases are usually some version of a cylinder or spinning-top shape... The idea suggests itself to build a giant ring-shaped starbase transporter ship, that fits around the starbase's vertical axis and docks with locking arms, drawing power from the starbase's generators.
There would be a ton of complications though. Starbases not having a navigational deflector is one, but also your transporter ring-ship would need to be efficient cruising at warp both by itself at low power, and cruising with both the added power and mass of the starbase docked in. Realistically given what a nightmare warp geometry and dynamics are, and also how you'd need to be able to draw close to 100% of the starbase's energy budget in flight, this would necessitate designing a starbase and transport system in tandem - it would very much not be a "plug and play" kind of system.
...But the attractions of being able to re-deploy starbases as needed are so tantalising...
Oh yeah, if we rolled sub-par on the Impulse as well, that could be a factor. If this thing would have been Manoeuvrability: High, between that and the plating, that's got to be worth half a rating to a rating. But in fairness, we'd probably have the same manoeuvrability if we'd stuck with the Type-2 thrusters, and at least this will mean it's ready for our other ships, so it's not all bad.
