maybe you can help me.
I am having the same issue, and did all you wrote here.
Here is my terminal output:
Filesystem Size Used Avail Use% Mounted on
devtmpfs 4.0M 0 4.0M 0% /dev
tmpfs 7.8G 1.5M 7.8G 1% /dev/shm
tmpfs 3.2G 2.1M 3.1G 1% /run
/dev/sdb6 29G 29G 418M 99% /
tmpfs 7.8G 16K 7.8G 1% /tmp
/dev/sdb6 29G 29G 418M 99% /home
/dev/loop0 128K 128K 0 100% /var/lib/snapd/snap/bare/5
/dev/loop3 114M 114M 0 100% /var/lib/snapd/snap/core/13425
/dev/loop1 114M 114M 0 100% /var/lib/snapd/snap/core/13308
/dev/loop2 56M 56M 0 100% /var/lib/snapd/snap/core18/2409
/dev/loop4 56M 56M 0 100% /var/lib/snapd/snap/core18/2538
/dev/loop6 62M 62M 0 100% /var/lib/snapd/snap/core20/1587
/dev/loop5 62M 62M 0 100% /var/lib/snapd/snap/core20/1581
/dev/loop8 83M 83M 0 100% /var/lib/snapd/snap/discord/137
/dev/loop9 163M 163M 0 100% /var/lib/snapd/snap/gnome-3-28-1804/145
/dev/loop7 83M 83M 0 100% /var/lib/snapd/snap/discord/132
/dev/loop15 37M 37M 0 100% /var/lib/snapd/snap/heroku/4092
/dev/loop12 401M 401M 0 100% /var/lib/snapd/snap/gnome-3-38-2004/112
/dev/loop13 82M 82M 0 100% /var/lib/snapd/snap/gtk-common-themes/1534
/dev/loop10 165M 165M 0 100% /var/lib/snapd/snap/gnome-3-28-1804/161
/dev/loop11 255M 255M 0 100% /var/lib/snapd/snap/gnome-3-38-2004/106
/dev/loop14 92M 92M 0 100% /var/lib/snapd/snap/gtk-common-themes/1535
/dev/loop19 1.1G 1.1G 0 100% /var/lib/snapd/snap/intellij-idea-ultimate/377
/dev/loop18 884M 884M 0 100% /var/lib/snapd/snap/intellij-idea-community/381
/dev/loop21 185M 185M 0 100% /var/lib/snapd/snap/postman/181
/dev/loop20 182M 182M 0 100% /var/lib/snapd/snap/postman/178
/dev/loop16 878M 878M 0 100% /var/lib/snapd/snap/intellij-idea-community/379
/dev/loop22 81M 81M 0 100% /var/lib/snapd/snap/robomongo/1
/dev/loop17 1.1G 1.1G 0 100% /var/lib/snapd/snap/intellij-idea-ultimate/375
/dev/loop24 47M 47M 0 100% /var/lib/snapd/snap/snapd/16292
/dev/loop23 47M 47M 0 100% /var/lib/snapd/snap/snapd/16010
/dev/sdb5 974M 201M 707M 23% /boot
/dev/sdb1 200M 14M 186M 7% /boot/efi
tmpfs 1.6G 136K 1.6G 1% /run/user/1000
du -hs /*
0 /afs
4.0K /bin
215M /boot
0 /dev
34M /etc
7.2G /home
4.0K /lib
4.0K /lib64
0 /lost+found
0 /media
0 /mnt
594M /opt
du: cannot access '/proc/4090/task/4090/fd/3': No such file or directory
du: cannot access '/proc/4090/task/4090/fdinfo/3': No such file or directory
du: cannot access '/proc/4090/fd/3': No such file or directory
du: cannot access '/proc/4090/fdinfo/3': No such file or directory
0 /proc
3.7G /root
du: cannot access '/run/user/1000/doc': Permission denied
du: cannot access '/run/user/1000/gvfs': Permission denied
2.2M /run
4.0K /sbin
4.0K /snap
0 /srv
0 /sys
16K /tmp
11G /usr
36G /var