Yes I have. This dtoverlay entry for fake kms is like default on Raspbian Buster (on Rasperry Pi 4).

But now that you mention it, I most probably was pretty confused by this guide over here:
Configuring-the-Raspberry-Pi

Following it I thought it would be good enough for fake kms to put the

display_rotate=3

in config.txt (as stated in my first post), but after disabling it (this is crucial!) and putting your suggestion about the xrandr option in autostart it finally worked in the end.

“In the end”, as I would really like it to be always rotated (including boot), but hey… this is something to work with.

Cheers!

-Corben