Hello. I come from the future. Not the really cool future with jetpacks, but the just-future-enough that this thread is a bit outdated.
TrueNAS’ documentation hasn’t caught up yet, but I can confirm that if automatic snapshots are enabled when creating an SMB share with the Time Machine purpose, a manual snapshot will be taken automatically when the backup completes.
Any snapshots taken using this method that are older than 7 days are automatically purged, even though they’re technically manual. The purging apparently happens as part of the snapshot process.
I’ve also added this thread to a master list of all my Time Machine and ZFS/TrueNAS posts, here: My Adventures with TrueNAS, ZFS, and Time Machine: A Master Post