If you've used Neovim vs Helix, I'd say the comparison is pretty close.
Tmux is more customisable, but a lot messier out the box. It's much older so has picked up a little cruft along the way.
Zellij is newer so has the benefit of being a clearer implementation. But it's designed as an "out the bix" experience, not one you can customise to your hearts content.
Edit: if you did want to try out zellij, I should point out there's a "no install, try in shell" option on their website's main page which is a super quick way to get a taste!
You can't rebind plugin key maps and creating a new session conflicts with my preferred custom mode bindings. It's clear the devs are both rather opinionated and want to ship features, but they gotta clear stuff like that up, I'm not interested in doing that from CLI when tmux does this without issue.
0.41 included some "non-colliding keybind" option, and you can clear-defaults in the config now, so this issue might be fixed. I haven't tested it but I recall seeing news about it.
Last I tried it the bind issue and performance were sort of blockers for me, but it looks like both of those are moving forward.
Tmux is more customisable, but a lot messier out the box. It's much older so has picked up a little cruft along the way.
Zellij is newer so has the benefit of being a clearer implementation. But it's designed as an "out the bix" experience, not one you can customise to your hearts content.
Edit: if you did want to try out zellij, I should point out there's a "no install, try in shell" option on their website's main page which is a super quick way to get a taste!