The squashfs version of 19.07.2 by default uses a f2fs overlay. This overlay is formed at boot time by an unusual process that splits the /dev/sda2 partition into the original squashfs (alias /dev/root) and a f2fs which starts at the end of the squashfs (alias /dev/loop0).
This makes it not trivial (though certainly possible) to expand the f2fs and have a larger overlay.
If you are storing a lot of data it would make sense to create and mount an additional data partition, then that data can be kept separate while upgrading the OS.