IMHO repair should be a feature of all shipyards.
Why? In contrast to the 19 need-it-to-build-a-new-hull shipyard addons, fast repair is a fairly significant strategic feature that seems like it might warrant having a separate building. Being able to get fast ship production facilities built far from your original system, but sacrificing repair ability might be strategically interesting...
And many of the other 19 shipyards should be combined.
We've got 21 shipyards and 34 build-able hulls.
Most of those hulls probably aren't presently interesting or distinctive either...
Shipyard upgrades should be significant and special.
To make them more significant and special, they don't need to be located at the same place as the shipyard. Instead, one such building could unlock stuff for every shipyard, or every shipyard within X distance / jumps.
At the very least unbuildable items should be grayed out or put under the "Unavailable" tab.
Something to that effect can be done. It's a longstanding missing feature, not intended functionality as is.
Alternatively or additionally, valid production locations can be highlighted when a producible item is selected, along with a display of range or what would be targets for a building's effects if it were produced at a selected location.
The difficult thing is the "explain in easily-understandable text what condition(s) this production location lacks or has that make it not a valid location to produce this item". Something like
location = And [
OwnedBy TheEmpire Source.Owner
HasSpecial name = "RESONANT_MOON_SPECIAL"
Not Contains Or [
could have 4 different reasons, or 15 different combinations of reasons why it makes a location not a valid production location (+1 case when it is valid = 16 = 2^4). A combined bit of handwritten text to explain all the requirements could be done, but marking which parts of that text are preventing production isn't easy.