Ok... finally figured this out. When you said use "the ext4 instead" I thought you meant the partition type rather than the image type. Utterly confused I gave up for a couple weeks.
Now that I've flashed the sd card using the combined-ext4.img instead of the combined-squashfs.img, things have progressed. Used cfdisk to resize /dev/mmcblk0p2 partition, then struggled to resize /dev/root until I found this workaround to an apparent issue with the x86_64 image on 19.07.07.
The steps at this link worked:
mount -o remount,ro / #Remount root as ReadOnly
tune2fs -O^resize_inode /dev/mmcblk0p2 #Remove reserved GDT blocks
fsck.ext4 /dev/mmcblk0p2 #Fix part, answer yes to remove GDT blocks remnants
#reboot
resize2fs /dev/mmcblk0p2
Now df -h shows /dev/root using the rest of the resized partition...