SvOlli

I advise that you do it the other way round: take a look at the download sections for stable distributions and buy one of those boards. A stable version of Armbian for the Orange Pi 5 might take months, years or never appear at all. All work here is done by voluntaries in their spare time, so don't expect anything, except you might have to learn how to do it yourself.
 
If you want something stable now, it might be worth taking a look at the official Orange Pi 5 software releases. I tested their Debian image and it worked so far, even transfer to and booting from NVMe. Didn't test graphics acceleration, though.

Is there any chance to get access to the gpios on the pin header again?
 
In /sys/class/gpio only two "banks" are listed "gpiochip378" and "gpiochip497". From searching around the net, I think that gpiochip378 is for configuring HDMI and gpiochip497 only has 15 entries. I'd also expected to find the gpios in "gpiochip0" as on other platforms.
 
Was this just an oversight in the device tree, when moving to a newer kernel or is it left out on purpose?
 
P.S.: for me it's not required to access the gpios via /sys/class/gpio, any method to write to pin 40 (aka RasPi gpio21) would work for me.

Because the kernel is running from the same RAM is used, but not counted. And also some RAM has been reserved for graphics. Using a buildroot similar to Squonk42's setup I've got ~56M of RAM available and ~40M free with just logging, a dhcp client and dropbear running.

The changes by "Squonk42" have been integrated into mainline buildroot something like two weeks ago. So if you checkout the current git, you can just build it.
 
I also did something similar using the external layer feature of buildroot. This keeps custom changes separated from the "official" buildroot, so hacking is a bit more straight forward and migration to a new version of buildroot is easier. It's available at:
https://git.h8u.de/Allwinner_V3s/buildroot-v3s
I can also create a mirror on github if anyone is interested in joining in.