Indexed shifting can cost a lot to add on, so I agree with your decision not to upgrade it. If you want indexed shifting, get another bike. You can keep this as a beater or commuting & errand bike. And yes, that rear derailleur will be able to handle any number of speeds.

Do you have a little bike computer? The cheap ones do a good job. Cateye offers decent value and has lots of satisfied customers, and you can get mount kits so you can move a computer from bike to bike.

Or you could get a smartphone app if you have a smartphone. They use a lot of energy from your battery, though, so they are not good for long rides. It would be a shame to have a discharged battery when you suddenly have to make an emergency phone call. I'm taking a long ride this coming weekend, and I plan to carry one or two external batteries for my phone.