I suggest backing up the pool, destroying it, and then rebuilding the pool once TrueNAS has been installed. That also allows you to ensure that TrueNAS features are natively supported (ZFS encryption, sVDEVs, and so on.) and if there is a issue, so what, start over, and try again because the data is safe. Upgrading on a production server with GELI while possible seems like many more opportunities for stuff to go sideways.
Patrick M. Hausen listed a way to decrypt GELI disks one by one, but to me, torching it all and starting over seemed like a much better idea. in particular, the sVDEV will not get filled with small files or metadata until you move said files around. So if your pool has oodles of space, you might be able to do that locally. I just thought it simpler to make multiple backups and then move the data back once the new pool was set up.
Obviously, there is a cost to redoing the pool, i.e. remaking all the relevant directories, and so on. However, I was also switching from an AFP-heavy/SMB-lite hybrid to SMB-only, so I had to redo all the sharing setups/ACLs/etc. anyway.