You could boot up into Single user mode and clean out the root file system.
Some suggestions would be, don’t run the Snap version of Plex, download the .deb install file. Don’t use /media for your storage mount points. It would probably be faster to start over with a larger boot volume or relocate Plex to a different drive.
Filesystem Size Used Avail Use% Mounted on
udev 1.6G 0 1.6G 0%
/dev tmpfs 343M 3.1M 340M 1% /run
/dev/mapper/Plex--vg-root 216G 216G 0 100% /
tmpfs 1.7G 8.0K 1.7G 1%
/dev/shm tmpfs 5.0M 4.0K 5.0M 1%
/run/lock tmpfs 1.7G 0 1.7G 0% /sys/fs/cgroup
/dev/sdc1 1.8T 1.6T 214G 89% /media/hdd2
/dev/sdb1 2.7T 1.9T 825G 71% /media/mainhdd
/dev/sda1 472M 208M 240M 47% /boot
/dev/sdd1 3.6T 2.4T 1.2T 67% /media/backuphdd
/dev/loop1 55M 55M 0 100% /snap/core18/1754
/dev/loop3 72M 72M 0 100% /snap/lxd/16099
/dev/loop2 72M 72M 0 100% /snap/lxd/16048
/dev/loop0 55M 55M 0 100% /snap/core18/1705
/dev/loop4 31M 31M 0 100% /snap/snapd/7777
/dev/loop5 30M 30M 0 100% /snap/snapd/8140
/dev/sdf1 1.9T 1.6T 278G 86% /media/networkdisk
/dev/sde1 3.7T 2.4T 1.3T 66% /media/travelplex
tmpfs 343M 8.0K 343M 1% /run/user/1000
