so I’m currently running 21.04.3 LTS
I think you meant 20.04 LTS, right?
I usually prefer to stick with LTS versions
There will be another ubuntu LTS release in two months: 22.04 LTS. Hopefully that release runs perfectly out-of-the-box on the Framework Laptop.
Any opinions on which one I should install?
There are official guides on both:
21.04 : Ubuntu 21.04 on the Framework Laptop
21.10 : Ubuntu 22.04 LTS Installation on the Framework Laptop 13 - Framework Guides
IMO, pick the latest version (21.10) so the transition to 22.04 LTS is easier.
Also: 21.04 is not supported by Canonical anymore. (As of 2022)