Previously you’ve used TWRP, now you do not seem to use it anymore.
What does this mean?
Is there still a recovery? (If so, do you ship your own recovery there?)
So do all advantages of Treble still work in /e/? Like OTA updates and installation in the background? (which Android seems to call A/B devices)
And what are all these images you need to flash there?
Sorry, for so much questions here, but can I then also still go back to stock ROM by flashing it or is this an one-way-process (and I could only flash other custom ROMs then)?
Okay, so, but as now edited in the OP, your installation instructions differ very much from the ones listed in the FP forum for GSI flashing. One does not need to select a slot, one does not install some APK overlay, one just flashes a lot off stuff…
Asked in different words: Could I also use the instructions in the FP forum to flash the /e/ image? And if so, what’s the difference?/what happens?
Nope, I want that information to be public, so others can later also check it. That’s the reason for having a forum. (That said, if you want to tell people they can answer here, feel free to do so.)