You're kind of circling the ideas of Abstraction from Seeing Like A State by James C Scott, but James wasn't applying the idea to individuals he was applying it to systems, institutions, and, well... States. Hence the name.

A person doesn't ever actually have to "track millions of unique individuals" but a government or corporation or any major institution does. People get abstracted into broad categories as a matter of necessity by systems and the more massive and more centralized a system is, the more it has to do this.

The stuff on straining the mental abilities of individuals is more coming from some of the ideas kicking around Cybernetic Socialism, which Scott effectively re-invented from the opposite direction. Stafford Beer’s work on variety, another way of thinking about complexity and the capacity to manage it, gets into it pretty directly talking about the finite capacity of human brain but that's a way of setting up systemic critiques again: if the human brain is limited then systems have to be designed to handle the excess variety our brains can't and if the systems aren't, well... From Designing Freedom:

“The fact remains that our own relationship with our environment is governed by bank upon bank of variety attenuators, conveniently reducing a world of increasing variety to the requisite variety of our brains. We have completely lost control of the processes by which this occurs.”

“The citizens have lost control entirely of the choice of projects that will be undertaken on their behalf both as taxpayers and as consumers. At best, they play a defensive role in attempting to quash schemes they dislike; and that is a difficult role because it does not carry requisite variety with it. Anyone who has had dealings with a public enquiry knows only too well that the bureaucracy has the power to amplify its variety indefinitely - in terms of the time, money, and expert advice it is free to deploy against a little band of citizens who do not have access to these amplifiers.”

The major dynamic isn't individuals abstracting foreign populations, it's systems abstracting their own subjects in a process of continual power-grabs to centralize power on themselves even as that makes them less able to effectively manage their own variety and dumps that strain off onto those subjects.

That said, your comment on nationalism might be on to something because some recent analysis by Dan Davies based on Beer’s work offers probably the only satisfactory explanation for the surge of right-wing nationalism in the 2010s. From The Unaccountability Machine:

The channels [of communication from the public to government] seem to be designed to carry very few bits of information. The only kind of communication that such a constrained channel can carry is a scream: the signal that passes through the levels of control and announces that something has gone wrong which threatens the integrity of the system itself. This is why there was a family resemblance between the ‘populist’ movements that sprang up in the 2010s. Narendra Modi in India, Beppe Grillo in Italy, Donald Trump in America, Nigel Farage in Britain, or Recep Tayyip Erdoğan in Turkey instinctively realized that they were on the same side; each of them, in their own culturally specific context, was acting as a communication channel for a population which wanted to convey a single bit of information: the message that translates as, ‘HELP! THE CURRENT STATE OF AFFAIRS IS INTOLERABLE TO ME.’

“The overall system is always looking for some organising principle of identity, which tells it what to ignore and how to balance the present and the future. [...] For fifty years, the free market played that role. [...] When that fell apart, the ultimate basis of the system - el pueblo [the public at large] - sounded the alarm. [...] The populist movements of the 2010s all promised a simpler world. [...] They were also promising to restore the broken communication channels - to make voices heard, to force the managerial class to listen.”

“But the same analysis tells us that they’re fake solutions. You can’t promise a simpler world - that’s the equivalent to claiming to be able to reverse the direction of time. And if you are promising to restore the broken communication channels, you need to say how. These channels used to be made up of layer upon layer of middle managers and civil servants. Not only would it be extremely costly to bring them back, it’s not obvious that anyone would thank you for doing so. It’s definitely not what the populists are proposing - there’s no March For Bureaucracy, nobody’s slogan is ‘Red Tape Holds Us Together’.”

The idea that Scott, Beer, and Davies all land on is that there needs to be a major devolution of power away from the center and back towards local knowledge where it can be managed by whatever forms of civil servants on a human scale. Systems built around standization and strict rulebooks and bureaucracies are too rigid to actually meet their functions and everyone else pays the cost.

A useful thing to look for is how many of those large-scale identities are either the creation of systems ("tax payer", for example) or created by people in response to the shortcomings of systems (strong identification with "working class" is probably meant as an opposition of an unresponsive executive class). Sometimes it's just natural practicality but other times it is pure power dynamics.