I have a fairly basic way of achieving this. I open three adjacent panes, work in the middle one, and resize them so that the middle one is just the right width for whichever app I'm using.

Unless the app I'm using is Vim (it usually is), when I use a ZenMode plugin. I use Neovim these days so use @folke's zen-mode.nvim plugin.

I zoom the tmux pane (i.e. maximise it) then enable Neovim's ZenMode. It's great for writing text.

I appreciate I'm answering this seriously late, but I'm surprised there aren't any answers yet, and I just found my way here via a more current Reddit thread.