I have heard the advice not to change the formation too frequently.
if I wrote that, I was meaning during game time, not season.

I have no idea where you got that from, I almost never play two games in a row with the same formation, I would if it was suitable for the opponents formation, every time I change the formation/tactics/players depending on the next match
agree
It's got to the stage now where I can tell, 90% of the time, in the first 10-15 minutes of the match I'm not going to get the result I want
yes, after some time, the manager has an instinct.