I know that OP ask something specific, but my L5 also says low disc space, both on rootfs and boot.
I have a very hard time to get enough space especially on boot. Is there a go-to solution for this?
I might have experienced this in the beginning, but I cannot recall what I did to solve it.
Edit:
Well, somehow “sudo apt autoremove” just solved my issue for now so I managed to get the phone up to date. However, I’m still interested what others experience.