Both Zirconia (2020) and Emerald have build parity and are current running eOS - 1.14.2 (Android 12)
Your phones should have been updated automatically. Here are the build download pages for manual upgrade option:
Zirconia page - /e/ image ROM download
Emerald - /e/ image ROM download
Issue trackers
Zirconia eOS issues
Emerald eOS issues
Has anyone noticed performance improvements with eOS as opposed to the stock Teracube 2e builds? If so I might take the plunge. I believe I’m running Zirconia.
Hmm… I never tested them head to head, and I only have one each of Zirconia and Emerald so it can’t be true 1:1, but each update that increases Android major version has been improving performance for both units, and /e/ performance is great by my non-rigorous testing, so I don’t think you can go wrong unless you know that /e/OS or MicroG don’t support an app you need to use.
What’s the recommended backup method for the stock OS? Do I need to root the device ?
You can’t really directly restore a backup from stock Android to /e/, so you should manually copy any important files off first and ensure other things like contacts and calendars are in cloud-backed providers.
Install instructions are here:
Zirconia: Install /e/OS on a Teracube Teracube 2e (2020) - “zirconia”
Emerald: Install /e/OS on a Teracube Teracube 2e (2021) - “emerald”
Terracube 2e has been updated to e/OS 1.14.2-s
NO COMPLAINTS 
v1. 15 should be out now, too! Give it a shot 
If I do, I may have reason to complain. 
As I wrote no complaints; yet.