I don’t schedule any friendly games during the first week then after that I arrange one ever 3 days and leave a week free before the first game of the season.
when it comes to substitution I like to sub off the players at half time because although I have in mind who my first 11 will be, I use preseason to fine tune my tactic and learn about the players so you might notice that in a particular situation player B will play a kind of pass that player A doesn’t and that seems to give you an advantage so even if player A was supposed to be my starter, because of this particular situation I might start games with player B or I make a mental note if it so if I need to bring him on as a sub to take advantage of the AI I know exactly when to do it. You only noticed that if you have seen all your players play enough games during preseason