How would you do Bastions?
The most important thing for a DM is to build the campaign around the concept of the players returning to their bastion often. If you don't prepare for this, the system won't work as intended.
I have been running a bastion based on the playtest rules for about a year and they work really well. Important to note: our bastion is on a ship, sailing in the MTG setting of Ixalan.
What are your underlying assumptions of what they should do?
It should give the players a clear and easy way to build their base as they see fit, with utility. Note that I don't really care for the gold coming in or going out.
Would it be an individual or group location and why?
I prefer group location, because I think it works better narratively. In my coming Sigil campaign, I might actually give the players the option to have individual locations.
How do you use it to drive stories?
For the bastion on a ship I am running, it is their transport. The stories come from the hirelings, which I run as a DM, so there are lots of little low-stake conflicts between the crew.
Same question as earlier? But it should give me a lightweight system I can tinker with. I prefer something simple that works over something complicated that might be better.
Should they be portable and how? (I don't know about you, but I'd save up good money for my own Baba Yaga's dancing hut).
Should? Maybe. I did it, and it's fun!
What I like is that the mechanicst only really happen once every couple of sessions, and there is no micromanagement. I still attack the bastion from within and without, so I ignore any notion that it's "hands off" for the DM. My players really enjoy having their own space and hirelings they can build relationships with.
For what it is, it works really well this way.