Man City has lost its best performance and it is caused by several players who are no longer productive. We can see and compare Man City now with the previous season and in the previous season all City players were very productive in making contributions. But this season looks like they have lost their enthusiasm to pursue the title in any competition. So having a busy schedule or having a shorter schedule will not have any effect on Man City's performance. Except for making major changes in the new season. So this season Man City only has to fight to maintain its position so as not to get out of the top 4 positions.
Maybe the players are no longer productive, or Pep's tactics are starting to not work. Man City still has big-name players. It's just that maybe you're right that they may not be productive anymore.
Looking at Kevin De Bruyne's performance after injury, he didn't perform as well as before. So is İlkay Gündoğan who can no longer play as strong as before. They still help Man City, but they are not in their top form at the moment.
However, we see Man City winning titles in several seasons and becoming one of the strongest teams in Europe. What we see now is Man City in decline.