My GPU is: AMD Radeon RX 5700 XT
A country wide population adjustment sounds acceptable to me. It won't be fully realistic, but it ought to get good enough results for large scale attacks even if things are off in individual cities.
My suggestion would that the first release of a population adjustment feature be the simplest possible version of such a feature (every pixel of population (do you have a name for your most discreet units of population?) gets a simple increase or decrease that is directly proportional to the country wide increase or decrease). But write it so it is easy to change later. Then, if you come across information on any historical trends for how rapidly denser population centers tend to grow vs sparser population centers then you can rework the feature so that the proportion of the population change that is reflected in each square of population is directly related to how populated that square already is (it is probably on some sort of S curve, with denser population centers increasing more rapidly than sparser ones up to a point and then leveling off). This still won't be 100% accurate, but ought to significantly reduce the error for individual towns and cities, and further increase the accuracy of large scale attacks.
Edit: Alternatively, is the resource that HerrTom provided compatible with whatever you used to generate the population map in the first place? How did you create that anyway? I assume it had to be automated in some way (feed the right data into the right program). I simply cannot imagine any human having the patience or lifespan to manually input the correct number for every discreet unit of population that you had. And mad props to you or whoever on your team worked on it if it really was done manually (actually, props no matter how they did it, that is an outstanding population map).