Honestly it's been a while for me too and I don't even know how to see the actual device partitions instead of just mounts (e.g. I think in this case your / is just a read-only ramdisk so it doesn't really matter what df says... mine is 100% too.)
I don't even know how this works anymore since I think since Android 10 they use some sort of overlay FS for updates?
I tried installing parted on Termux but it refuses to do so and I have no energy to fight it.
Or I may just get a new phone the next time I need to do Android. This one has 12, they're at 15 now. I may need to be on whatever's the latest on a new project, whenever that comes (not soon).
I don't even know how this works anymore since I think since Android 10 they use some sort of overlay FS for updates?
I tried installing parted on Termux but it refuses to do so and I have no energy to fight it.
I know it's cliché but... have you tried a factory reset?