I found this looking for the same thing. Basically My window is 281 columns wide. I'd like for tmux to choose to split along the vertical axis (create side-by-side windows) and balance those until splitting them would create a window that is too narrow (80 columns in my case). Instead I get very wide but short windows which for looking at CLI results is less optimal I feel.

I wonder how hard this would be to code as a new layout?