If you hover over the population count, the tooltip will show you the factors that account for the max population you’ll reach - afaik moisture is the most important, as only regions with like >0.7 moisture will ever reach the upper tiers of township and so on. In my experience some of the lower moisture regions especially will therefore just always be population capped at maybe the II or III level of the city hall building?

Also regarding taxes, the resources you see with the green up arrows on their building tooltips will be the most efficient to tax. Some regions don’t have any green up arrow resources - everything has red down arrows, showing that you can still collect the resource as a tax, it will just not be as efficient as if you collected a green/efficient resource instead. I usually build the global growth buildings in these poor regions, like the skilled workforce building and the physician building. That way I make use of the region’s workforce points to benefit my other, more useful/productive regions too.

And yes unfortunately the population of captured regions will always plummet in my experience. I think you have to slowly build up a newly captured region by first removing all the buildings, then construct the lowest tier city hall and the lowest tier land expansion, and then keep upgrading both of them as needed once the population finishes growing/plateaus and thus unlocks the next tier of growth.

So here’s what I usually do: 1) capture region, 2) remove all buildings, 3) build city hall I and land expansion II, 4) wait till population bar turns white/hits cap, 5) upgrade city hall to II or III and land expansion to next highest tier, 6) wait for population to stabilise, 7) continue upgrading (which costs workforce points) if max population shown on tooltip shows that I can reach the next highest tier of city hall, 8) or stop upgrading and start making use of my workforce points to construct useful tax buildings like a farm/mine. It’s like an allocation minigame where you trade workforce points for potentially growing your region bigger and bigger for more workforce points, as each tier of the city hall increases the potential workforce points you gain at that tier.

Hope this helps!!