I believe housing in FFXIV was designed for players to experience their neighbors unique ideas and inspiration for their housing decoration. Instancing housing entirely, because of this, seems like the wrong idea for me.
Housing in 14 was designed specifically for FC's.

Your proposed idea would make it harder to do any of the things people use FC's for (Have fun trying to get multiple people to sit around in the workshop every time you want to send out ships/subs, especially if you arent sending them all to the same place, no matter how large your FC is) and solve more or less none of the issues the housing system has.

FC's arent struggling to find houses, FC's are struggling to find Large houses.



Make everything instanced, keep one ward of each housing district as the hub zone for those instances, populate it with more vendors to turn it into a true hub city, make garden patches usable inside and bam, you fix almost all the problems housing has.