So this did not really turned out as I expected.
I did not get anything posted above working so I just cancelled all thoughts about Gateway and bridging and forwarding and other überkomplex stuff that may or may not have been relevant and took the simplest possible approach.
What can go wrong, right?
Give the WAN (eth0) DHCP, hook it up to my Switch. Full stop. Nothing else.
My UISP server scanned and found the EdgeMAX Router.
My UISP server was able to “adopt” the EdgeMAX Router.
My UISP server allowed me to assign the EdgeMAX Router the role as “Gateway”.
From there I could configure adoption of the rest of my UI devices, some more not visible here,:
Everything works as I wanted. It was as simple as that.
Now, how this affects anything else is yet to see, but I am happy for now. 
Lesson learned, sometimes things can be a lot easier than expected.