I think it’d be good if players could determine which locations go into which province, but they’re incentivised to group locations by culture or have a mix of cities and rural locations. What’s the benefit of having fixed provinces?
Maybe this would also be possible through reforms, for example, Austria changed its internal borders the most during the era of enlightened absolutism, or when Maria Theresa ruled. It reduced the number of quarters within states such as Styria, Carniola, Upper Austria, etc.
There should be some dynamics, there should be rules depending on the periods or how much administrative level of development you have. The game should force you, just like the real world, that the provinces should not be too big and not too small.
One obstacle to not having too large a province would be the number of inhabitants, let's say that at the beginning of the game your administrative development allows normal management with 100k inhabitants and that the province functions with a minimum of 15k inhabitants. This means that if you exceed the maximum allowable number with your province, you have the problem that these taxes flow into the "black market" as a percentage of the excess. If you do not have a sufficient number of inhabitants, the administrative costs become more expensive according to the percentage of the deficit.
Another criterion would be the number and distribution of locations, for example, the capital of the province develops the best, each location further away from it the development of locations would decline, for example, the first would be 85%, the second 50%, the third 15% and the fourth 5%. Thus, it would also force the player to place the capital as much as possible in the middle of the province, which could also lead to regional centralization.
IIn addition, one of the obstacles must have been that you can make administrative reforms only for a certain period of time. Even if you did it the first time, you would have a smaller drop in stability, maybe -10 points, but if you repeated it twice in a row in the same area, for example within 10 years, the drop in stability would be much larger (-30 or more), because people often over time unify their identities within the province and such an event upsets them. One of the obstacles would also be the type of government, feudalism, tribalism, etc. should not implement administrative reforms.
Another matter would be interesting, when the capital of a country becomes too large in terms of population, its location could be the province itself.