Once again, thanks for your nice guide!
Just extended my root partition by system_a as you described.
I think it’s worth mentioning that you get messages such as /dev/mmcblk0rpmb: read failed after 0 of 4096 at 4186112: Input/output error. Anyways, each step was confirmed with a positive status message in the end. So I ignored those input/output errors and everything worked fine (so far).
[nemo@xa2 ~]$ df -h
Filesystem Size Used Available Use% Mounted on
/dev/sailfish/root 5.2G 1.9G 3.3G 36% /