Thanks for your replies.
I finally discovered what the problem has been-i do not have USB-C on my computer, so i used the cable that came with the 6A together with an adapter i had, that was the reason why nothing worked.
I bought an USB A to C cable and that worked pretty good.
My first impression is really good, sadly it seems not to get VolteHD and Wifi Calling when the network is bad, but that is not a problem that belongs to GrapheneOS, the same behavior can be observed on the stock rom.
What makes me wonder a bit is, i get this "not an original firmware" (or something like that) screen when booting the phone, but that dissapears and then everything boots normal. Did i do something wrong or does everyone get this screen?