See this is why the slider thing would work so well. It would/could be based on tech practicality, shipbuilding capacity, leadership, whatever fits, etc.
The slider concept is a decent idea. But it won't happen because it doesn't fit at all with the rest of the production system.
I think Paradox kept the build times to 730 days for gameplay reasons - players are impatient people - so realistic build times are probably a matter for modders.
What I'd suggest is a basic 1100 days buildtime for a battleship which can be modified downwards to 900 based on technology, Practical values, politics, ministers, and/or National Decisions and perhaps upwards to 1300 based on the same. Reducing the buildtime could increase the total cost in some instances.
A x2.5 increase would be fairly historical but probably way OTT in gameplay terms - except for any mods that give the USA an evern more mammoth historical IC.