You are right. Bayer Leverkusen are stronger than Brest obviously, even though Brest will be playing at home, and they will want to do everything possible to win against Bayer Leverkusen, but it will be something difficult for them to win against Bayer Leverkusen. Brest are not in good time either, so even if they will be playing at home, Bayer Leverkusen are going to be ahead of them regardless. As long as Bayer Leverkusen will perform the same way they are doing in the Bundesliga, I don’t think they will find it difficult to beat Brest.
Besides that the quality of League 1 and Bundesliga is different and as far as I know Bundesliga is superior to League 1 so I think Leverkusen is more favored to beat Brest but still we don't know what will happen in that match because Brest also can't be underestimated in the Champions League they performed well but in the domestic league they are not so good I am also surprised and I think Brest is more focused on the match in the Champions League so Leverkusen needs to be careful when facing this club they could give a strong fight especially since they are playing as the home team.
Indeed, the conditions of Ligue 1 and Bundesliga are very different, in Ligue 1 it is always dominated by one team, namely PSG and for several other team it is only like enlivening it because apart from PSG no one can perform consistently and stably so that the competition is not really competitive.
In contrast to the Bundesliga which is filled with more strong teams in each competition, even the Bundesliga team has more experience in European competitions which makes each team much more experienced and difficult to be dominated by one team in more structured way.
That way, Leverkusen quality will be more guaranteed than Brest, even though both of them won in the previous 2 matches, Leverkusen is still the team that can have better game to beat Brest away.