I think I want to take a lot of inspiration from plan9's acme. When you first open the editor there will be a title bar across the top of the window which a number of useful tools typed out, but able to be changed by simply editing the heading. For example:

save select snarf cut paste pan 
-------------------------------
            .-   .-.
        .-.(  )-(   )
       ( .   (   .   )
      (  (    (___(_.))
     ..________..\    ..
 .-.                        .-.
(__.)               =      (__.)
                    |\
                   /|o\
          ~~~  ~~~/_|__\~~~~ ~
                ~~ˋ~~~~ˊ~~
            ~~ ~     ~~~~~~~ ~

Instead of having pencil line box tools you could right click an individual glyph to "select" it. If you right click something like ---> instead you've selected a line tool that uses - and ends with >. You could instead right click and drag over only two of the -- to select a plain line without an ending character. Or <-> to have a double ended line. For boxes you could right click and drag a box over something like:

╭――――╮
│    │
╰――――╯

I think this would be a super easy to use an intuitive system. Anything kept in the heading of the file will not be included in the saved file so it's a nice place to store your palete and the tools like snarf which allows you to select a region and copy it with left click. Middle clicking to paste perhaps? I'll have to think about the commands a bit more, but the idea is that select is the default mode which you spend most of your time picking and drawing in. Maybe it should be called draw instead if the right click operation is always "pick".