Going from Bayern to Tottenham is a step back rather than one forward. Yes he would be in the Premier League and it is all good, but there is nothing to be won with Tottenham now.
Going from Bayern to Tottenham is not a step back. He's not that big of coach to say he's too big to coach Tottenham. Coaches like Mourinho and Conte that have achieved a lot coached Tottenham so why is Nagelsmann too big for Tottenham? Bayern put a lot of faith in him when they hired him because he has never done a big team let alone be successful in one. Nagelsmann is a good coach but I cant say he's up there with the likes of Pep, Ancelotti, Mourinho. He will be a perfect fit for both Tottenham and Chelsea so if Tottenham actually want him they have to move things quickly and get him before the summer.
Precisely this is an opportunity for him to show his best qualities. It is very difficult for Tottenham to win titles, so he has to bring Tottenham to win titles. It will be a very extraordinary achievement for him to bring a team that has been considered difficult to win in any competition. I agree with you that Nagelsmann is a good coach, but he can't be equal to other top coaches in the Premier League.