It was some experimenting, but I managed to get the PRFS working at boot. Also the Samba server and ProFTPD are started at boot. So now after putting on the power, one can use it directly.
During experimenting the RPi stopped booting after I modified /etc/modules and /etc/fstab. The easiest way to get the RPi working again was to take the SD card and connect it to (through an USB card reader) to my laptop. After starting Linux in Virtual Box and have the USB bridged to Linux, I could access /etc/modules and /etc/fstab again and commented out the changes.
There were two unexpected things that were different than one would expect. Firstly the modules should be copied in /lib/modules/6.1.21+ and not in one of the subdirectories. Also one should use kmod and not depmod, by creating a symbolic link.
I have updated the building instructions.