If you're up for a less elegant alternative, you can use the iOS app
FE File Explorer. It's actively being developed and maintained, and supports SMB, SFTP, FTP, WebDAV, among other protocols.
In the app, you create an entry for an SMB share (such as
mainpool/iphone-photos, hosted on your TrueNAS system), while also giving File Explorer permission to access your
Photos. You can then transfer photos from FE File Explorer to your SMB share. There's more to the app, as it supports a few other features and nifty conveniences. The only issue is that it does not track what photos were already transferred. (Unless there's a way to keep
Photos in sync with the SMB share, but I have yet to discover that.)
The
Pro version costs $5 (one-time purchase, no monthly membership nonsense), which allows you to create multiple entries without a limit for only one SMB share. If you try the app out, play around with the free version before you commit to anything. I went ahead and dropped $5 for the Pro version since I use it for many things, not just Photos.