I like to have a setting that functions on its own with or without the players interacting. So when they do, I know the ramifications.
Also, as a natural source of complications they can engage with.
I've used "An Echo Resounding" before as a system to do this and would like to discuss other options and how people made them work.
Where do you find people that are interested in that sort of thing and have experiences to share?
I prefer system that start with a design that rests how things worked in history. Then if it is a fantasy setting layers the magic on top of that. My personal system is a hodgepodge of material from History, Harn, and GURPS. A little from the Mythic Europe source material from Ars Magica. And more recently a bunch of stuff from Adventurer, Conqueror, King. In the Axioms Newsletter the authors explain the basis for their mechanics which allowed me to tweak and adapt them from first principles.
The foundation of what I do rests on agricultural productivity with various charts and lists about how long it take to clear, build, and grow stuff. From that I can figure out the surplus. From that I can figure out the sizes of a region's military forces and any other extras like magic school, temples and the like.
I try to reduce everything into straightforward mechanics no more complex than the world building stuff in Traveller.
I highly recommend Harn Manor for when a player owns an estate. Although I am going for something much simpler for my rules.
ACKS for something easy to manage an entire realm.
I like the Pathfinder 1e Campaign Book for construction as it about OK if you spend X then this is what you can get, this is how it helps you, and this is roughly the size the result will be. I am in the processing of adapting it to my own rules.
The idea behind my approach is to learn to detail. Figure out the range, figure out the average. Then boil that down into set of charts. I leave the option of going backwards to more details if that what the players want to do.
Players want to know couple of things
1) How long it will take to build
2) How much it will take to build
3) What it will take to maintain
4) What income do I get from it.
5) What benefits other than income are there?
6) The rough size so a map can be draw of what was built.
If you can do all that without a spreadsheet than you nailed it.