But In early 80s the Korean and Chinese players were lacking middle-game strength compared to the Japanese pros, so the focus was on the middle-game.

So I still think that, as amateurs, the middle-game is the area which has by far the biggest payoff in terms of effort to learn vs. results, but one shouldn’t neglect fuseki.

Back in the 90s I had some lessons with the Shutai Zhang who been through 4 years of pro training in China but didn’t turn pro.

He talked a bit about his experiences, and I definitely picked up that in the chinese training in the early 80s, the middle game was the main focus. Oversimplifying a great deal, the Chinese players waited for the stones to come together before the game really started with the middle-game fighting.

In the 80s Japan-Chin matches in Go World, you see games where players such as Jiang Jujo (against Awaji) and Nie (against Cho Chkun) make moves that were regarded as clear fuseki mistakes (wrong choice of joseki).

Maybe over the intervening years the emphasis has changed in China to the fuseki.

When I was insei in Japan the fuseki comments were quite rare (not more than “this is the usual move”), the focus was definitely on the middle game.
It may have had to do also with the fact that I was always playing just “boring” fusekis on purpose, to save the thinking time for the fights.

I think that your statement about one being able to improve the middle game by reading exercises is an oversimplification.
Getting a feeling for the direction of play with fighting groups in early middle game cannot be done by reading; reading only helps in close fights.

Another way of thinking about it is: the middle game and endgame you can improve on your own by doing known, widely available exercises to improve your reading and counting. The experience of a pro is most valuable in the most poorly understood part of the game—the opening, so if you have access to a pro, it’s good to use his/her time for getting advice on that.