It is impossible for Tuchel to underestimate this match because after all, Galatasaray is a tough opponent for them, so the slightest mistake could lead Bayern Munich to defeat. Bayern Munich will definitely not waste the opportunity to get full points in the group phase and I am quite sure they will play with a core squad to avoid defeat. Galatasaray also has experienced strikers in the team so Bayern Munich must be quite wary of them.
Bayern Munich did not play in their best condition when facing Manchester United because even though they were playing at home to Bayern Munich, Manchester United managed to score three goals and Tuchel had to prepare a solid defense in this match to stop Galatasaray's attacks.
Bayern might be a little more confident as well as Galatasaray who have a strong mentality after they were able to win over Manchester United, actually this is a match between two teams who have a chance of qualifying from this group, maybe they will have more motivation to be able to get points in the match This.
However, realistically, if we look at the quality of the two teams, of course Bayern is much stronger so I think they have a bigger chance of winning the match.
I believe in Bayern Munich and indeed in my opinion this is not a match that is too difficult for them, while if we look in terms of performance it is quite clear that Bayern Munich is superior to Galatasaray. And besides that in this match Bayern Munich will play at home by hosting Galatasaray, so clearly they should have a greater chance and also high confidence in this match. If we look at the group board, this match is only to prove who is stronger to get the first position, not to fight for tickets to qualify in the group stage, as you said none other than because these two teams already have a great opportunity to qualify for the round of 16.
Apart from that in terms of strength, maybe we all know that Tuchel's squad is superior, especially with Kane as a new scoring machine there. I honestly can't predict how many goals he will score but I'm sure they will be easy enough to dominate this match and end up with the full three points.