I cloned Debian-9 to Debian-10, upgraded to Buster. VM using 7912.65 MiB disk.
Followed instructions here:
https://www.qubes-os.org/doc/template/debian/upgrade-8-to-9/
Template trim procedure is:
$ sudo fstrim -av
$ sudo shutdown -h now
(wait for template shutdown)
(start the template)
$ sudo fstrim -av
$ sudo shutdown -h now
(wait for template shutdown)
(continue your tasks that depend on creating VMs from or cloning the template)
Doing it twice ensures that any significant changes from earlier in the previous session aren't stored in a difference file.
Brendan