Is there a company that provides upgrade kit? At all? Afaik it's impossible due to complexity, even google had something similar in mind but dropped the idea bc it's too hard
https://support.fairphone.com/hc/en-us/articles/360047776791...
Also a different form factor, but look at what Framework laptops are doing. You can upgrade every part, including the mainboare, individually. If it's possible for an ultrabook form factor, it should be possible for a slate handset form factor (IMO)