OK, after sleeping on it overnight, here are some ideas.
I think that both the shipyard and the slipway should be granular. You cannot build higher than the lowest of both of them, and the total hull spaces of units in the shipyard cannot exceed the shipyard's total.
Granularity should be around 10HS not 1 HS, 1HS makes it too fiddly.
My only problem with this whole idea the Electric Boat Shipyard. During the early 1980s, it was building Los Angeles class subs(~6900 Tons), Ohio class subs (~18800 Tons) and refitting Sturgeon class subs (~4800 Tons). One Shipyard, with 2 slipways, and a pair of drydocks.
Pete
We should split the current shipyard into 3 separate items.
Shipyards are the main area, that is where the "industrial work" gets done.
Slipways are primarily designed to build new ships, but they can overhaul, repair, and refit ships at a penalty (25% maybe). I added overhaul in the list for forward bases. I have a shipyard with a slipway, but no maintenance facility (possibly due to combat damage) and this way you could overhaul a ship using the old method.
Drydocks (or spacedocks) are the third item. They allow overhaul, repair, and refits at normal cost, but building ships is at a penalty (again maybe 25%)
One of the reasons I suggest this is the US Navy during WWII Pacific campaigns had "Destroyer tenders" and "Floating Drydocks" forward deployed to fix battle damage.
The drydocks were modular, like we were talking about.
We then could then add 2 "ship items" called "Tender" and "Drydock"
Tenders allow repairs of battle damage like Damage Control without using up replacement parts. You could also be alongside and perform a minor refit to replace the replacement parts.
Drydock or "mobile spacedock" would allow quicker repairs of battle damage and quicker minor refits. (replace the replacement parts at double speed maybe). drydocks are large, and cannot be mounted on a ship with engines. You MUST tow them. Which forces you to develop a tug.
Pete