I do not think that this will go on forever. We always said the streak will end with Lewy, Müller and Neuer. And this season, with Lewy gone, Neuer injured we weren't far from it. That we scored 18 of the 20 goals with Tuchel when Müller was on the pitch and only two when he was not doesn't make that thinking less unrealistic...
Bayern is not really a comfort zone for Kane. If you do not win in Munich things get very unpleasant. And Kane would go from a team with less than 50% ball possession to one that has to be dominant and has a lot ball possession and the other team parking the bus. He has to adapt his game to that.
It would mean that he goes out of his comfort zone at Tottenham into another country, into another league with other rules and from the little pond into the "big pond" were he is not the homegrown kid that always has the sympathy of the fans but has to first earn them. He can learn a lot there.
For the English national team that actually could be a big advantage.
You're right to suggest that the Bundesliga represents a new professional challenge for Harry, and although you didn't say this you could probably have added the personal aspect of living in Germany while his kids are still young and the positive development aspects of that for his children. Harry is already set for life financially and will hoover even more wealth that he can't possibly spend down into old age wherever he goes so that's not a factor either.
That said, even though Bayern had a pretty poor season by its standards it still won the league and got through the CL stage comfortably. Harry can help push Bayern forward but if fails to meet the expectations set by Lewandowski and other Bayern greats he'll have a rough go of it, as you also suggest. So in that sense it will be a challenge, but only a challenge to maintain the same and quite frankly boring league win streak (is it 11 or 12 straight now) that Bayern are at very little risk of losing grip on with or without Harry.
It may come down to what his wife thinks, but if I were Harry I'd stay in England. The only problem with staying in England, however, is that it makes little sense to remain a Spurs man (unless he wants to walk on a free next season), City have no use for Kane, Arsenal are out of the question, Chelsea are a dumpster fire of a club now, and Liverpool don't seem on the surface a good fit for him, and despite everything I just don't see Newcastle a good fit for Harry...which leaves United, which on some level is a perfect destination for Harry but then again, we are a badly run club and we're in no way expected to seriously challenge City for the PL trophy in 23/24 unless we address ALL of our obvious needs this summer, which is far from a certain outcome.