I like the expanding map idea, but I worry it will lead to situations where players will have to rebuild bits of their factory in order to get stuff to the new section, as they've built a lot of spaghetti in the way or something. Why not have a system, where you build a set level of production in ...

Brilliant work - but I think you've missed something that a lot of people do. Your 4x4 balancer would not work, for reasons shown in this article: https://wiki.factorio.com/Balancer_mechanics.
You actually need 6 balancers to do it correctly.

How this impacts your golden role, I don't know, but I ...