if the USSR population weren’t de facto serfs, why was travel so restricted?

That’s not what a serf is, by any definition. There are LOTS of reasons why travel was so restricted. One of them was because the vary majority of the population were serfs under the Tsar and literally could never afford to fly. Another was because commercial air travel wasn’t really a thing around the world until the mid-50s, basically after Stalin died. Stalin restricted travel in the decade preceding the war with the Nazis, basically right around the time Mein Kampf was published where Hitler openly declared that his goal was to invade Russia, dismantle the USSR, and enslave the population. There was a LOT of resentment over the revolution and there were many people who were sympathetic to the fascists and even collaborating with counter-revolutionary forces outside of the country in an attempt to build a movement to restore the Tsar.

But the BIGGEST reason for travel restriction was, and continues to be for all countries that do it, economics. Brain drain is a real phenomenon, and wealth distribution in the world is skewed to the imperialists. Brain drain is the result of a structural process of stealing vast sums of wealth from the rest of the world and then bribing all the best and the brightest from the exploited countries to move to the imperial core, which further impoverishes the exploited countries. There’s really no way to stop that unless you restrict travel. If you don’t restrict travel, international economics basically dictates that you will always lose talent to the imperialists. Travel restrictions are essentially nation-level non-compete restrictions.

Why was the wall built?

You mean the Berlin Wall? In Berlin? Or do you think the Iron Curtain was a literal wall around the entire USSR? The Berlin Wall was built because of the absolute MESS of the situation in Berlin. The Red Army had defeated 80% of the Nazi forces, marched fully through Berlin, and liberated many concentration camps. But the imperialist allies demanded that they get to control part of Berlin. So the Soviets controlled East Germany, but the imperialists had half a city INSIDE East Germany. The Soviets built the Berlin Wall to completely encircle West Berlin.

Think about that for a second and then ask yourself why the literal physics were such that the Soviets built a wall around West Berlin to contain it but you and everyone you know (including me) were raised to believe that the wall was built to keep people from leaving the USSR? Why, when it is just so obvious that the wall was built to contain the imperialists?

To me, that was some strong evidence that many of the things I had believed about the whole situation was deeply suspect and highly propagandized. It drove me to actually research as much of this stuff as I could.

Why are there so few accounts of people fleeing to the soviet union?

Because even years after the worst economic times of the USSR the majority of people polled say that the dissolution of the USSR was a net negative for them and their families? https://www.reddit.com/r/communism/comments/649fe1/25_years_later_polls_in_eastern_europe_show/

Are you really claiming that some dairy farmer in Turkmenistan could decide one day that he instead would like to serve borscht in Moscow for a living, and then just get up and do so?

I’m sorry. This is just so funny. Do you realize that Stalin was the poor son of a poor shoemaker and a laundress in Georgia and became the General Secretary in Moscow? Like what even is this question? Yes! Literally tens of millions of poor farmers all over the USSR moved to the cities for a better life and found it. That’s literally what industrialization does.