RockBian

The file /boot/armbian_first_run.txt is sourced by a bash script (/usr/lib/armbian/armbian-firstrun-config), but only if the file /root/.not_logged_in_yet exists.

Long shot. In the wiki on the corresponding screenshot you have the choice between /dev/sda1, /dev/sdb1 and /dev/sdc1, which are all partitions. Your sda isn't partitioned. Nor is your md0, but maybe the algorithm simply skips all device nodes which does not end in a number.
So put a partition on your disk.

$ cat /etc/apt/sources.list deb http://deb.debian.org/debian buster main contrib non-free #deb-src http://deb.debian.org/debian buster main contrib non-free deb http://deb.debian.org/debian buster-updates main contrib non-free #deb-src http://deb.debian.org/debian buster-updates main contrib non-free deb http://deb.debian.org/debian buster-backports main contrib non-free #deb-src http://deb.debian.org/debian buster-backports main contrib non-free deb http://security.debian.org/ buster/updates main contrib non-free #deb-src http://security.debian.org/ buster/updates main contrib non-free $ cat /etc/apt/sources.list.d/armbian.list deb http://apt.armbian.com buster main buster-utils buster-desktop

$ cat /etc/apt/sources.list deb http://deb.debian.org/debian buster main contrib non-free #deb-src http://deb.debian.org/debian buster main contrib non-free deb http://deb.debian.org/debian buster-updates main contrib non-free #deb-src http://deb.debian.org/debian buster-updates main contrib non-free deb http://deb.debian.org/debian buster-backports main contrib non-free #deb-src http://deb.debian.org/debian buster-backports main contrib non-free deb http://security.debian.org/ buster/updates main contrib non-free #deb-src http://security.debian.org/ buster/updates main contrib non-free $ cat /etc/apt/sources.list.d/armbian.list deb http://apt.armbian.com buster main buster-utils buster-desktop

The boot.scr script is damaged. You can try to recreate it. Boot from sd once again, mount the emmc filesystem and recreate
 
mkdir /tmp/mmc mount /dev/mmc/mmcblk1p1 /tmp/mmc chroot /tmp/mmc mkimage -C none -A arm -T script -d /boot/boot.cmd /boot/boot.scr  
 

On my system (kernel 5.8.17) a thread kworker/fusb302_wq was running, permanently eating 18% of one core, and causing the load average to be always at least 1.0. The module fusb30x could not be unloaded, rmmod just hung, although it's use count was zero. After blacklisting the module and rebooting the problem was solved.
Now is the question: what did I disable? Some googling gave me this document: 
So it has only to do with the USB-C port? The serial connection is still working fine.