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.
I agree with you that the quality between Ligue 1 and Bundesliga is different and that puts Bundesliga better than Ligue 1, so it will also affect this match, so it is very natural when Leverkusen is favored to win when facing a team from Ligue 1. Although slightly declining, Leverkusen's performance is still very good, especially since they also bring great motivation after successfully becoming champions in the Bundesliga last season. This season is the most anticipated season for their performance. Many people hope they can play very well and can talk a lot in the Champions League this season. The expectations for them are high, so Xabi Alonso as a coach will also try to answer those expectations.
Both teams played very well in the Champions League but if you look at the opponents I don't think it's tougher than Leverkusen because this club is facing clubs that usually compete in the Champions League so I think Leverkusen will beat Brest no matter what or at worst get a draw.
For the chances themselves Leverkusen has odds @1.50 as a club playing away I think that's a pretty good chance to bet on but we also need to be prepared if there is a surprise Brest itself has pretty high odds as a club playing at home that is @6.40.