Hello,
I tried with another Android version, and before the update “Update & Backup”, the data partition is 11GB.
After running the install.sh and rebooter script, I have a 4GB system partition again.
By cons I found the following link: [How to] Debian Stretch + Linux 4.9 from scratch (w/download, EMMC only)
but I do not know if the method is still operational, and is compatible with Armbian.
Sincerely Marc.