I've found it surprisingly hard to find a summary of the differences.
By contrast learning German from Danish seemed to have some bits that made it clear reasonably fast.
And can't you just start watching TV shows and movies in Mandarin? It should be a lot easier than for us mortals who start out without knowing any Sinitic languages. We have to work hard just to handle the tones and the near total lack of shared vocabulary.
I am puzzled that learning Mandarin for you hasn't gone much the same way as learning German did.
There is of course the standard way of playing with HelloChinese (or similar apps) and LingQ/Du Chinese (or similar apps) + reading easy readers (Mandarin Companion, Chinese Breeze, and similar). You should be able to speedrun them, compared to the long slog path that we mortals have to take.
(A fellow Dane with zero East Asian parentage.)
Cantonese and Mandarin weirdly seem to have wandered further from each other than Danish and German. At least for me, but that may also be lack of exposure.