Thanks for pushing this forward!
Even though I am by no means experienced in the topic, There are some aspects that you didn’t include in your post.
While designing the geo node caching system, I think it is relevant to consider two additional future projects.
The first project is the Shader Caching which has been discussed in detail in the Texture Node’s discussion.
The design of those systems should resemble (as much as possible) IMO.
The second project is the Edit Mode node which AFAIK means that there would be a node that will cach the current geo node state and will additionally allow to edit/sculpt it as a regular mesh. (Maybe even editing values in the spreadsheets?)
Another thing to consider is the difference between saving to disk and caching. Although similar, those systems have different use cases, and they may be linked together or not. Of course, this kind of thing may change between caching systems. For example, the requirement and use-cases for caching a texture (rasterization) or saving it to disk (baking) may differ from their parallel options in geometry.