Aftermarket springs seem generally not as well made as factory in my experience, not sure why. Maybe find the OEM maker and get the size/rate you want from them? Old porsches, my old D90, went back to original on everything. Isolators in the D90 and Gwagon are nice for a bit of lift/cushion while keeping stock springs.
My P38 rides on factory air - everything can be kept working right pretty easily and just check the system regularly with a soapy spray bottle. Just o-rings and plastic tubes, all pretty basic stuff that's fun to fix. Haha. Can't beat the ride and the ride height variation capability define these cars. Would never go to coils.
If I were to consider a mod it would be to Arnott Gen 3 - which are progressive in the opposite way of original air. They are a bit tighter at cruise height and are softer when raised - this was a design flaw of the originals in my view, but I don't really want a harder ride at cruising height. But I might try it if/when the second set of factory bags on her break (they've been replaced once).