It seems we're on our own with this problem. Either no one has an answer or all knowledgeable people never had to deal with this problem.
We also choose a not-so-popular phone. The tablet-phone combi is too expensive and the tablet of mediocre specs.
I went for the too-good-to-be-true price for the phone-only... and after a lot of reading found out that quit a few people had problems with their phone.
Perhaps some code monkey (no disrespect) could tell me if we have more options when the phone is connected to a Linux system? Android is Linux based, so that would be a compatible pair. Windows can only see one partion on a flash card and, but I might be wrong, Windows cannot read EXT 3, 4 partitions.
Windows device manager does recognize the phone as "ASUS Android Bootloader Interface" so the phone is not completely non-responsive.
I also posted on a German site; Android-Hilfe where there's a dedicated forum with knowledgeable people so perhaps that gives more options.