thanks for the reply, i have a RT-AX58U, with the same version, when it is readonly, on this system it's not 100%:
adminklaas@RT-AX58U-8410:/tmp/home/root# df
Filesystem 1K-blocks Used Available Use% Mounted on
ubi:rootfs_ubifs 80164 70388 9776 88% /
devtmpfs...