I assumed this would be your answer. How do you deal with the fact that in Gittin 76b, the students of the academy would not even go out to escort their friends? And in Kiddushin 31b, Rabbi Yohanan was unsure whether to permit Rav Assi to leave in order to go greet his mother. That is lighter than a trip and it was prohibited, and it is much more reversible than a trip and nevertheless it was prohibited.
Do the explanations that these were stringencies, or that they were priests, persuade you?

That indeed could be merely a stringency. After all, amoraim did go abroad and return. There are always explanations about Torah study and so on, but those explanations seem dubious to me.
But beyond all that, in a situation where there is concern that going abroad may be irreversible, there is logic in setting a sweeping prohibition even on reversible cases. In a situation where no such concern exists, there is room to permit reversible cases.

There is always concern that going abroad may be irreversible.
It is not reasonable that this was merely a stringency, because if it were merely a stringency there would be no place for Maimonides to prohibit every departure. Besides that, many medieval authorities explain in the passage in Moed Katan that leaving without permission means a prohibition on leaving because it is for an ordinary purpose; it is clear from their words that it is forbidden to go out cruising around.

A sweeping prohibition does not become permitted when reality changes, unless you say the reason has lapsed, in line with the Rosh that we mentioned a few days ago (in the question about gentile milk), but here the reason is certainly not clear (it could be that they were concerned one might settle there, but it could also be that they were concerned you would become enamored with abroad and the value of the Land of Israel would diminish in your eyes).

As for amoraim who went abroad, it is obvious that this was for Torah study; I didn’t understand why that seems forced to you.

There is always concern, but not always at the same level. In our circumstances today, leaving is not a significant act, and so there is no reason to prohibit it except in the narrowest possible way.
The number of departures does not create the impression that this was always done for justified reasons. After all, one can study Torah in the Land as well. If this were really a clear prohibition, it would have been appropriate to avoid it except in cases of absolute necessity.
By the way, in the Gittin passage you mentioned, the case is about people of the Land of Israel escorting Babylonians who had come to study in the Land of Israel. How was it permitted for them to return? What is the permission for living in Babylonia? They came to study in the Land of Israel and then returned to study in Babylonia?
And in Maimonides too, the wording is problematic. He writes that it is always forbidden to leave, but it is permitted for commerce, “but to dwell abroad is forbidden.” That implies that the prohibition is to dwell there. Similarly with his words about returning to Egypt, while he himself lived there. Regarding Maimonides’ position on this, see here: https://www.machonso.org/hamaayan/?gilayon=42&id=1278
And in the Shulchan Arukh this prohibition is not explicitly brought at all.

One can study Torah in the Land, but the academies in Babylonia were better. How was it permitted for the Babylonians to return? Everyone asks that; apparently they prohibited departure only for residents of the Land of Israel, and that also explains Maimonides and the other sages who visited the Land and then left.

As for Maimonides—it seems to me that his words are very clear. Leaving temporarily is forbidden except for marrying a woman, and so on, and for commerce, but dwelling abroad is always forbidden.

Anyone who thinks the Torah of Israel should be separated from the Land of Israel and the people of Israel would be better off leaving abroad and settling there.