It sounds good! Good luck.
>> two small patches where I can build up a bit but can use mostly the native soil and amend it.
I think that's smart. Focus on a small area at first, and learn what works best or is the easiest. Don't rush or burn yourself out.
Expand next year or at leisure. Acquire compost when it's convenient. Decide what amendments work best with your light clay. Maybe figure out whether the white cementy soil really is construction material or bad for plants, or whether it is a sandy subsoil that would greatly improve your clay if you mixed 20-30% white stuff with 70-80% clay and then amended that with compost.
Maybe plant a few delicate plants in 50% white stuff mixed well with 50% clay, then see whether they die or thrive.
