The problem of systemd is that it hasn’t been just a replacement of init as they initially claimed
Apart from the PID 1 part of systemd, almost all tools are optional.
Although I have a positive opinion about the systemd project, I used netctl instead of systemd-networkd for a long time without any problems. And even today I don't use systemd-resolved because I use a combination of unbound and Pi-Hole in my private LAN. And so on.
So you can't say that the systemd project has replaced various solutions in such a way that you don't have a choice anymore.
Changing the label should also work during operation.
For example, it should be sufficient to run
sudo btrfs filesystem label / newlabel
to change the label of the root partition (/) tonewlabel
.If I'm not mistaken (can't test it right now), you have to specify the mountpoint when the partition is mounted. And in unmounted state the device (e.g. /dev/sda).