"Then one day I saw an advertisement for a gadget that attaches to a guitar neck and plays chords at the push of a button."

I was going to make a joke about how if you left out the guitar neck, that would be an accordeon (I didn't say it was funny), then I suddenly recalled the autoharp. That might actually fill the bill, although I think it does require dexterity, but not as much as fretting a guitar needs, and if you're not Bryan Bowers.

We used to have a Chromaharp but my Dad used to like giving things away, so one day he did. I would say I played with it rather than that I played it, but I sure wish I had it now.

I don't have an autoharp but I do have a concert zither, which wouldn't help you. However, I can say that tuning zithers isn't too bad. Mine stays in tune remarkably well. I never have to retune in the course of a day. If you tune it every day, you only need to adjust a string or two each time. If I don't play for awhile, it takes about 15 minutes to tune it. I always forget, I think it has 63 strings. Thank goodness for electronic tuners!