I am trying to update 3B image to run on 3B+ but it needs to install packages worth 480MBs. They are to be installed on /var/cache/apt but I only have about 360MBs free on that partition. You can have a better understanding by looking at the image attached The sd card has about 3Gigs of unallocated memory that I can use to expand mmcblk0p1 or p2 but I don't know how to expand root.union.
It seems it is not
/ mount point) you need to expand but
/deb/mmcblk0p2 partition (
/ro mount point).
You should be able to resize the underlying FS. The following links may help you:
/ro should be mounted read-only (I assume this, but can't know for sure since I miss your
mount command output) I think it could be easy to do.