Ciao Sam,
Getting to the Fattoria di Maiano isn't super easy with bus, as the buses don't go exactly up to the estate... but #7 does go more than halfway up, and with a short 4 minute walk you can go from the bus stop the the farm. Or if you want to skip the walk, you can take the bus all the way to Fiesole and from there get a taxi, it should be a shorter ride and thus cheaper. The problem of course is that most taxis at most fit 5, not 6 people. If you want to book a taxi van, all you need is to call 0554242 ahead of time and ask for a taxi that seats 6 people to come and get you where you are and take you all the way to the Fattoria. It won't be super cheap because you always pay for the call but it will avoid having to deal with bus schedules.
If you want to try the bus, it isn't hard - the problem where people run into is not buying their tickets BEFORE getting on the bus. If you do that, then all you need to do is get on bus and put it into the machine near the entrance (check what other people getting on before you do) and get that ticket stamped: it prints date and time, your ticket is now valid for 90 minutes from that time. You can ride all the buses you want, doing transfers, within those 90 minutes.
To see the bus schedule, know that ATAF is no more - the region unified all bus companies across Tuscany into one single company to make it easier for everyone to use and find out where to go, as well as having one ticket for the entire system instead of tickets at each city. The city planner on the new website lets you put in your departure and arrival and tell you what buses to take, as well as what stop to get off:
www.at-bus.it
Unfortunately, that still needs work because it interacts with google maps which isn't super updated with the new company (it still links to Ataf) and doesn't give you the name of the bus stop where you need to get off (it is the stop in front of the Villa San Michele luxury 5 star hotel villa). You walk back to the last corner and take that small road. I've walked this road to get across below Fiesole and it is quite narrow, there isn't a sidewalk... it is fine to do during the day, because super careful and aware of cars. I wouldn't do it in the evening or with poor light.
Bus tickets cost €1,50 for those 90 minutes, if you stay longer you'll need 2 tickets per person. So total cost is €3 per person, total would be €18 for the trip. Taxi most likely would be more, but you can't beat the convenience. A taxi fare calculator I found estimates around €13 each way - you can check it out here:
https://www.taxi-calculator.com/taxi-fare-estimation
You can have the fattoria call you the taxi when you are done.