Good morning @Ingo_FP_Angel. The OP’s initial problem solved - Fastboot on Samsung is an issue.
Our three documentation sources present TWRP install on Samsung as quite a challenge, imho, for users new to this.
doc.e.foundation Starlte install
HowTo Linux Samsung /e/ install
HowTo Windows Samsung /e/ install
The Starlte install includes the wording: “Samsung devices come with a unique boot mode called “Download mode”, which is very similar to “Fastboot mode” on some devices with unlocked bootloaders.” Sorry, but I don’t have the experience to argue with some devices.
When I read my first comment here and the way that I tagged you in, and look at the Install TWRP source, I see that the OP’s TWRP link is actually quite newly edited. The way that I made my comment now looks insensitive. For that insensitivity I apologise. I had not logged in on Sunday, and seeing this thread I felt perhaps I could add something for the OP in a couple of minutes before an early start.
Now I think you are driving at improving documentation? I am certainly in favour of that and will share my very limited knowledge if you ask. 