I like the idea, but feel the changes necessary would be too drastic, and would cause as many problems as it solves. It's always easier to turn what you have into something better, instead of starting from scratch.
I have a few thoughts I'd like to share, and then I'll get into how I think your ideas should be implemented.
Heavy handed approach
I agree that it would be beneficial to keep players closer together, for the reasons you mentioned; activity and accessibility.
I think your idea is a good foundation, but forcing players to stay closer together removes choice, and effects a lot of edge cases.
What if you wanted to set up an outpost to scout out the enemy before a siege?
What if you want to establish a new holy site for your religion in a very specific location?
What if there's a resource you absolutely have to capitalize, regardless of political consequence?
An ideal solution would allow settlements to spread out, but encourage players to play more closely together.
I assume this was the intention for activity checks.
Resource based economy
In the real world, things like gold, iron, wood, have value because they can be turned into things that are useful, or because they can be used in a way that is useful.
For example, a chunk of raw iron has very little value on it's own, but by refining it, forging it, and turning it into a tool, value has been added in a way that makes the finished product more desirable.
The effort of all these processes, and the skills required to carry them out, creates scarcity, which drives up cost. If I could find the tool I was looking for from a short stroll in the woods, it would hold no value, because everybody would just do the same.
My issue with a resource based economy in a game is that items have very little value to start with, and it is hard to add value to items in a way that makes it consistently desirable. In real life, the creation of a tool relies on the experience and effort of countless people, but in a game, I can mine the iron, chop the tree, and bring it all together in just a couple of minutes.
Think about how Mina are created, used, and destroyed, and how precisely all these things can be controlled.
My idea
Have few 'capital tiles' that have a high upkeep in Mina, and require settlements to vassal themselves to one of these 'capital tiles'. Then, charge settlements an upkeep in Mina based on their distance to the capital.
At the beginning of a world, the Humans may group together around a single capital, building their settlements closely together to avoid paying high fees. Small settlements that do not pull in much Mina would inevitably want to stay closer to a capital, and large settlements that are quite successful would be able to shoulder the higher fees.
As a nation, outposts would still be viable, but limited in number.
As a settlement, space and distance would be a serious consideration.
As an individual, roleplay would be more accessible, and settlements that are far away are guaranteed to be active.
If a settlement is unable to shoulder the fees, it falls into decay, and may be claimed by another group willing to take up the mantle.
A large settlement may chose to become a capital itself, if they are able to pay the price.