Well, if I need to resize those rigid panes, I'll just have to suck it up and manually move my hand to the keyboard ! ..and exercise is supposedly healthy anyway 🙃

I'll give it a spin. Thanks a lot.

edit: git had good information: https://github.com/zellij-org/zellij

edit2: "Try Zellij without installing

bash/zsh: bash <(curl -L zellij.dev/launch) fish/xonsh: bash -c 'bash <(curl -L zellij.dev/launch)' "

VERY easy to test out!

Last edit: https://zellij.dev/documentation/overview

Zellij and Byobu? Byobu is less pretty but more functional, probably because it's a front-end for existing mature technologies (screen/tmux). I've seen it described as more or less just a list of presets for tmux, so presumably you could get the same effect by spending a few hours customizing base tmux.
Zellij feels new and hip (special fonts for pretty borders and buttons) but some aspects don't feel quite finished (like how it's missing multiple modifier keys to avoid keyboard shortcut collisions, how it's not in major distribution repositories, no drag-resize panes (that I've found, at least)). It feels inevitable that Zellij is going to overtake the incumbent multiplexers and soon but for the moment I just really wanted that system monitor status bar.
Both have panes, tabs, sessions, degrees of gpm functionality. Zellij has the other-user-focus indicator, Byobu has the status bar and renamable tabs. Byobu will automatically reattach if a session is found, Zellij needs a few lines in .bashrc first. Both can save and load layouts but Zellij needs you to write the layout to a kdl file somewhere while Byobu can do it with one keypress (not sure if that survives across sessions though). I get the impression that permanent config in Zellij requires more manual config file manipulation, which isn't necessarily a negative, but feels more like having to learn a new programming language for something you'll set up once and then immediately forget about.
Both have more pros than cons and I'm sure I'll continue bouncing back and forth as I refine my understanding and opinions.

A quick thought experiment for how I think Zellij could close the distance:
Preset layouts, or community layouts that are easily shared
Live system resource monitor status bar (I spent weirdly long looking for a single-row live terminal resource monitor that I could just set up in its own pane and just couldn't find one)
Of course, being able to bind multiple hotkey modifiers, though I've read on the github issue that that's not necessarily a trivial feature)
More mouse functionality (though I might be in the minority there, haha)
Maybe a TUI interface for setting config items

Looking at this list, it's really just a bunch of things that would suit my own personal workflow, haha.