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!
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!