I had the very same problem. Especially, I could not jump to the string I wanted to copy. I wrote a tmux plugin for it https://github.com/schasse/tmux-jump Maybe it helps you too. Works great with https://github.com/tmux-plugins/tmux-yank

It's been one of my #1 terminal frustrations, too!!! But instead of relying on GUIs and the mouse, I started to use terminal tools and wrote a plugin for tmux that helped me. https://github.com/schasse/tmux-jump Maybe it's useful for you.

A fast way to jump wherever you want in your terminal without using the mouse. tmux-jump is written in ruby and can easily be installed via tpm.