yes the stress caused by the national championship or other challenges certainly tires the team which will then be much more approachable by the other teams that will challenge it, it is certainly difficult to play all the competitions in a row, but it is like this and it is necessary for the big clubs to have two teams to compete at high levels in all the competitions
More precisely not two teams, but reserve players who are as good as the main players in the club so that the club does not experience a sharp decline in strength when there are several main players who are injured due to playing optimally in all competitions including in difficult competitions such as the Champions League. However, the problem for some clubs is that having reserve players who are as good as the main players is not easy because players who have good talent will certainly be very reluctant to sit on the bench.
It only makes sense when those reserved players are alternated more often so that they keep their forms and confidence. In a case where a team has reserved players that are as good as the starting XI or almost as good as them and rarely use them they begin to lose form and when the coach decides to use them after exhausting their main option those reserves end up underperforming. Some players get insured because they have been over used whereas they had substitutes whom the coach would have given a chance to prove their skill while the other maintains fitness.
This is the real job of a coach, especially with a very tight schedule because of the many competitions that make players tired because of the little rest time that leads to injuries. The coach must often rotate players, in addition to maintaining player fitness, this is also effective in making substitute players more confident because they get more playing minutes. To find core players with the same reserve players is certainly difficult if they don't often get playing minutes because they don't get chemistry but when they are often played, reserve players will get chemistry with each other.
I'll take two examples of coaches who often rotate and those who don't often rotate players. Guardiola is a coach who often applies this, that's why they are often consistent every season, and Ancelotti is a coach who doesn't often rotate, so it's no wonder that last season many Real Madrid players were injured. So actually this is where the coach's function is besides formulating strategies, he must also be smart in rotating players so that they are consistent in every match. Moreover, the big teams that play in the Champions League with a new format like now, of course the match schedule will be even more.