> The problem is that pbpaste and pbcopy do not work under tmux

They do in recent macOS/tmux versions. You don't need reattach-to-user-namespace anymore.

These are my keybindings for copy/paste to clipboard:

    bind-key -T copy-mode-vi 'v' send -X begin-selection
    bind-key -T copy-mode-vi 'y' send -X copy-pipe-and-cancel pbcopy
    bind-key -T copy-mode-vi MouseDragEnd1Pane send -X copy-pipe-and-cancel pbcopy
    bind-key -T copy-mode-vi MouseDragEnd3Pane send -X copy-pipe-and-cancel pbcopy

`prefix + ]` then pastes using the clipboard contents