The competitive level of the Bundesliga is different from the English Premier League, here I believe that Sadio Mane will be easier than when he was with Liverpool. Even if he doesn't play in his usual position, I don't think it will be a big problem for him. Maybe it will be a little different when it comes to the Champions League, but in the Bundesliga he will be able to do well.
I also think that the competition in the bundesliga is still far below the premier league in terms of competitiveness, as far as we know the title competition often only occurs between dortmund and bayern munchen and obviously bayern munchen in the end is more dominant to come out as league champions, now lewandowski has left bayern munchen but I don't think it will affect the productivity of Bayern Munich goals too much later, because Mane is also a striker who is quite reliable in scoring goals, Mane has been playing in the competitive football league so far, so when he come to the league was not too competitive of course it won't affect it too much and let alone make him mentally down.