I didn't think we could do that yet.
Not through raidz expansion. I had built a test rig with 6x2TB to work on testing the expansion code (and then, shame, never did), so I'm adding another 2x2TB to that test rig, will run it as a raidz1 (12.2 TiB usable) and send/recv my video files and sundry stuff in there (11.4 TiB), and throw away the backup dataset (4.4 TiB). Then rebuild my main raidz2 - likely on TrueNAS 12 which means I am then not going back to 11.3 - and send/recv the other way, recreate backup dataset manually and do a full backup on the three Windows machines that run into it.
The test rig is actually my main Windows machine, I just threw a HBA and a bunch of disks into it and a Linux dual-boot. I'll run TrueNAS Core from USB stick for this send/recv, should be fine for the amount of time I need it. No Windows machine while that's all going on :).
Some caveats here, my husband loves keeping old backups around so there are some files we need to figure out what to do with. I think they're useless at this point, haven't been touched in years, and, who knows. I think best option is to rope him into taking a look at those old backups and really determine whether there's anything in there he needs, and if so take just those files out.
So, why now? Well, I am at 75% used. I can hang on another year. And, there's no guarantee raidz expansion will be here in another year, and the longer I wait, the more challenging it gets to fit my data into the test machine, or a single 14TB drive. It already won't fit into a single 12TB. I debated just adding one 8TB drive, which will be enough for the next 3 years I think - but then I didn't think I'd get to 15.8 TiB used in 1.5 years, so screw it, I'm going for 8. That's the amount of SATA ports I have, it doubles my usable storage, and if that's not enough, I have a problem and should see Storaholics Anonymous.