It will work, but unfortunately, it's not going to be enabled, and the way that it's set up you can't enable it. (Perhaps @bonienl will adjust the plugin in the future to not have the restriction that a SSD cache must be installed for the UI to appear)
What you want to do is create this file on the flash drive
/config/plugins/dynamix.ssd.trim/ssd-trim.cron
# Generated ssd trim schedule:
0 0 * * * /sbin/fstrim -a -v | logger &> /dev/null
Followed by this command:
update_cron
(This'll run the trim everyday at midnight)
Alternatively, since you don't have a cache drive installed, it's just as simple to install the user scripts plugin, and have a script that is simply
/sbin/fstrim -a -v | logger &> /dev/null
And then use user scripts to manage when it runs. (Doing it this way you can also uninstall SSD trim, as there's no requirement for it to be installed)