I only use 3 players in my friendlies for financial reasons (yes, I also have an old man of 200 -0.2k- salary lol) but I understand your complaint.
I think your suggestion is valid, but it would affect the financial aspect and this makes the matter a little more complex.
That's why I leave mine for this subject: Possibility of determining the minutes of each player on the court only in friendly matches.
Since these are worthless games, I think it's only fair that game time is determined by management rather than a player's performance. This would "protect" the teams that pick 5+ players against the ones that pick 4- without having to affect the financial aspect of the teams.