thetraveller1 I suspect the tablet will be easier. Even though Google took time away from shipping Android tablets, Samsung and others have been doing it continuously and, I suspect, feeding changes to Google over that time, so AOSP may already have lots of the foundational code for a tablet. I suspect much more of the folding-phone code is bleeding-edge, and parts of it may take a while to migrate from Google's private OS code into AOSP, and then longer for it to work well.
As just one example: any changes GrapheneOS has made to the lock screen might now need to be debugged on two or three different lock screens - the little screen, the main screen, and the unfolded joint screen (those of you who are folding-phone experts can easily detect that I'm not).