Hey Steve,
I am also getting very frustrated with the caching feature not being available inside Resolve 15's version of Fusion. To the point where I've re-created some comps in fusion 9 rather than suffer through the crashes and frustrations I've been dealing with inside Resolve. The one saving graces I've been using in place of that feature is the Saver node which will let me render sections as .exr sequences. Then I work around and re-import those sequences into Resolve through the edit tab into the compound (or Fusion) clip I've created and back into Fusion - what a pain - learned my lesson though
Made a note to myself to just go back to the old VFX connect workflow instead of trying to do it all in Resolve, it's a nice idea but perhaps I just don't understand the workflow enough or it's not polished quite yet.
As for your initial question, I don't do much 3D in fusion, but could you just create a blocker object with a 'holdout' type material which will serve the same function as the mask. It won't be as flexible, but it will allow you to block your 3D objects without rendering into 2D space - just my thoughts
Best of luck
-Matthew