Ciao!
Bus 12 and 13 here in town run similar route in opposite directions. So to get to Piazzale Michelangelo you might take bus 12; if you want to return the way you came, I would suggest crossing the street and at the bus stop on the other side take bus 13. Because of one-way streets the route will not match perfectly but the idea is to get you to around the same areas.
You can also take the same bus and continue in the same direction until you get back to your original bus stop.
Remember that bus tickets here in Florence work by TIME stamp: they are valid for 90 minutes from the time you got on the first bus and "activated" it or, using Italian terms, "validated", the ticket for use. The machines near the doors are for that reason: put your ticket in and date and time are printed on the ticket. You can then use the ticket for the next 90 minutes, so if you are ready to leave Piazzale Michelangelo check your ticket. If you still have time on the ticket, you can still use it.
When you are the "end" of the line, the bus driver just turns off the bus and generally takes a break, usually off the bus so he or she can have a smoke or make a call. Break time will be around 5 minutes, sometimes less, sometime more. So if you want to keep on going, just stay on the bus. Once the driver returns, he just turns it back on and continues the route.
Looking back at your original questions, both buses have stops right around Piazza Tasso and it would be more convenient time-wise to take the #13 back rather than doing the whole circular route. Both have stops at the Piazzale AND San Miniato so you can get off at either. Just so it's clear, they are two separate stops, San Miniato is a little further behind the piazzale so you can get off there and first go see the church, then just walk down to the Piazzale, roam around, then from the Piazzale get the bus back.
The new AT Bus website uses Moovit for their own "trip planner" so maybe they are in partnership with them and time schedules are more up to date on there than on Google Maps, I haven't made a comparison yet. Will have to do that soon! Let me know if I answered all your doubts, and if you have any other questions, happy to help

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