Simply create your render template BEFORE adding it to a job. Transcoder can’t use modified templates that have not been saved (unnamed). Set your fps and save template (click small button with Diskette icon)
One of the options is Snapshots tool from Vegasaur extension (commercial). It comes with Take Snapshot 1-Click command that you can assign a shortcut to (in Customize Keyboard window). All your snapshots are collected in the Snapshots window. At the end, select the desired snapshots and click Export button.