PSG's move to focus more on the Champions League is very appropriate because in Ligue 1 no team has been able to surpass PSG until this season and Brest itself will also be quite overwhelmed when having to face a team like PSG who in general have been much more consistent in playing in Ligue 1 this season. In addition, the next match for PSG is to be the home team with Brest as their guest and that certainly will not make PSG afraid because PSG is always better when playing at home with any opponent in Ligue 1.
Focusing on the champions League may not really be a bad idea but at this point I don't agree that it should be a primary focus for PSG as it may turn out just putting efforts at points where you may not really get the kind of productivity you expected especially as the team is having to deal with reshuffle that happened in the team over the transfer window recently, it will be more productive if they rather focus on the team build and making up for a few vacuums they have got and probably an intensed attempt in subsequent season would work much better than this season, although there's no limitations to the much they can achieve in a. Season especially if they put their mind to it depending on their strategy and tactics for the season.