I agree more or less ...

I think that the amount of Unrest reduction per unit should be less than 5 % ... maybe 3 %.

Also I think that unrest reduction should only apply to troops that were trained in your cities (meaning that I would exclude all troops that were gathered from outside of your cities, for example wildings or mercenaries). Why? Because I think that the latter troops are rather irregular troops, which are less disciplined and, if usedfor policing duty, might cause as much unrest as they reduce (for exmaple by trying to extort money from the population or abusing their position of power in other ways)

As for population of cities meaning something:

Would definitely be nice. Another idea (that might be used with or without your ideas) would be, to see population as the cities hitpoints.

With this system a group of monsters attacking a city wouldn´t raze it immediately, as soon as you lose the battle, but instead they would inflict as much damage (in population count) as the sum of the attacking values of all (remaining) monsters in the group (or double/triple the sum).

This way large cities would be able to last several seasons till they are razed and even small cities could survive for a long time till they are razed to the ground, if the number of monsters that made it through your defenses is rather small.

Same mechanism could apply to enemy empires trying to raze your cities. So even 2 single Juggernauts would take 2-3 seasons to level your city to the ground.

The problem with this is that there would be very little incentive to producing the unrest reducing buildings.

It would be much more efficient to produce units as it would both lower unrest but also increase your power rating which has value in itself for diplomatic purposes and obviously military purposes.

With the suggested 0.1%/lvl you would need 10 lvl 10 units to replace a belltower. Sure it raises your powerrating and why shouldnt it?

Basicly you have a full army you pay every turn standing around instaed of sacking enemy cities, conquering the wildlands or continuing their training. The main use imo would be to counter the 5-15% differenace your cities have depending on how many essences they have (cleric) so that you can adjust tax rates to keep 0% unrest everywhere.

I would agree so long as it's only to suppress unrest caused by things other than taxes - Antipathy, leader with the Cruel trait, Black Market, that sort of thing.