As of right now, it does not appear to be possible to index files that live on a TrueNas system in Windows 10 using the file explorer. 3rd party metadata tools, such as Sound Miner can index files though. The result is that searching for files from a Windows client PC can take a long time.
This workflow doesn't work: https://www.online-tech-tips.com/windows-10/add-network-folder-to-windows-search-index/
The dialog box to index the mapped network drive does not appear. See attached photo.