I think its hard to have a foldable device that has a phone aspect ratio (2:1) when open and a reasonable size (and thickness) when closed.
This could only be done like the flip phone (1:1 --> 2:1) or by multiple folds (like the triple fold huawei).
You could go for the 8600G/8700G APUs without a graphics card. This should give you better performance than the Steam Deck and the option to later add a graphics card.
I’ve added a screenshot of a german shop that sells these for 649€ without operating system:
I did not find any pre-built PCs with this CPU, that’s why I’m referring to this shop that does the assembly for the customer.