Moreover, it will be very difficult to judge on one match, and the result is expected as a rule immediately, fans are not patient
True. Fans are the worst judges there are. That being said, judging a national team coach is quite difficult because the number of games is limited, unlike a club where they can see the progress every week or every transfer window. A national team needs to produce results befitting their image in most competitions. Then again, that's the job of a functioning sports association. The bad news is we don't even know if the guys running the association are competent or not, most of the time (at least in my country).
Yes, here we need either a whole tour of friendly matches so that the coach can build a normal tactics of the game, or a few games at the official level within some tournament. Honestly it's hard for me to imagine what a new coach should do when he is expected to produce results in the first game, considering that the players are assembled a week before the national team game. What should he teach them? This is not the national championship, this is the national team level, there is too little training.
The issue about friendly matches is that you can never create the same tension and atmosphere for the players. It is either because your own players don't take it seriously enough or because the opponent doesn't play their best game. If you really want to know whether a certain strategy or formation works well against good teams, the only way is to find out to test it during at least the qualification games for the big international events. In friendly games you can see weird things happening that do not very well reflect what should be expected in important games.