Sysex librarian. Havent tried it yet, seems to work for some people here. ( search this forum and/or your search engine) Also I mention beware of data and saving structure on a4 ( soundbanks and projects seperate). But you said it worked with c6.
Or visit a friend who has windows or mojave with a usb stick and a bottle of wine.
Good luck have a nice evening
Edit: librarian, not liberarian
And thats for backup, transfer should work for os update in catalina.
Edit : seems maybe not, someone was given advice to use librarian for catalina os upgrade too, when he had a checksum error on model sample.