it was inherent to the terminal that you can’t position the cursor and select text using the mouse, and also inherent that there are not right-click menus

There may be terminals for which that is a limitation, but with neovim in konsole, I can (left) click to move the cursor or select text and right-click to get a contextual menu.

Separately, the TUI libary we use for RESTMan is Brick and does support mouse events. IIRC we only support mouse-wheel scrolling, but there's a lot more that is possible.

So, I think it might worth your time to try some TUI alternatives to your GUI environment.

Your larger point that TUI can't fully replace GUI stands, at least IMO. Even for plain text formats (e.g. dotty) that I want to edit with a fixed-width font, an image (pre)view can be essential to some tasks. I don't usually find that's true for coding tasks, but design docs often benefit from charts.