It‘s been a while, since I setup my bpi r3. But first of all, we need to solve your space problem. I think the easiest way, would be the get the sqashfs factory image from openwrt.org. Then head to: system/ firmware upgrade in luci and install the image. If I remember right, luci puts the image in /tmp, which is a ram disc. After the reboot everything should be fine again. On the fresh install, install cfdisk. With it just resize /dev/mmcblk0p5 to occupy the remaining free space and reboot afterwards. After the reboot you should have 8GB space available.
Edit: the post was saying: resize /dev/mmcblk0p66 before, which is wrong. I posted the correct procedure below.