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.