It's not massively unreasonable on the surface of it. A main defence line is easier to move than a whole city. The argument is there. But also not giving up land is also not unreasonable when you consider the examples of what happened when you gave in with Czechoslovakia and Baltics, especially when that land includes parts of that main defence line, making the whole line kinda worthless. Czech Sudetenland also included their defence line, so it's understandable that that example would be fresh in people's minds.

So really there were no guarantees that after agreeing to it, giving up the main line, the USSR wouldn't just take the rest of it. It clearly had ambitions in Eastern Europe, in the former Russian Empire's lands and had just divided Poland with Nazi Germany. So if you gain anything depends on whether you believe USSR had further ambitions in Finland or not.

Comparing this to Czechoslovakia, USSR still took exactly what it initially demanded

That's just a result of USSR wanting a quick conclusion to the embarrassment that was Winter War. Had Finland folded in the war like they had hoped, I doubt they would've settled for just those areas. They had puppet government ready, were planning that they'd do this and that once they win and so on. See the Baltics, it started out smaller then they were absorbed.

asked USSR for protection (because Nazis were scarier), Soviet troops entered those countries and suddenly there were Soviet state institutions in place and plebiscites.

No. USSR compelled them to take in troops with ultimatums, same ultimatum Finland got. Baltics and Finland just chose differently, Finns to fight and Baltics to give in. But once the USSR's troops were in, it was basically over for the Baltics and those troops were used then to conquer and absorb them.