You can always create a service override (/etc/systemd/system/plexmediaserver.service.d/override.conf)
and specify TMP to be wherever you want it.
I know most distro installers think root only needs to be 16-32 GB. (I run 128 GB).
(You can customize the size of root when you next do a fresh instal of the OS / create the partitions … ( /boot/efi, /, swap, /home)
something like this:
[chuck@lizum Downloads.2013]$ df
Filesystem 1M-blocks Used Available Use% Mounted on
/dev/nvme0n1p2 124870 30533 94338 25% /
/dev/nvme0n1p4 789911 541984 247927 69% /home
/dev/nvme0n1p1 499 26 474 6% /boot/efi
As for moving TMP,
