Ciao Joanne and welcome to our forum!
I'd like to offer an honest response to your request and tell you that you are trying to do too much in one day. Which part is the most essential for your visit? How many days total do you have? I am just trying to see whether you would have a chance to spend more than 1 night out in the countryside.
You can certainly join in on day tours from Florence that will take you to wineries and do tastings and tours. Those are easy to find on
Viator, such as these (
affiliate link) but, in that period, I would highly suggest spending more time in the area so that you aren't trying to fit wine tour + tastings + lunch + general experience into a day. It would just be easier to accomplish spending maybe two full days out in the countryside, without being a rush.
As for the harvest experience, two things I'd like to point out:
1.
timing of grape harvest: it can start early in September and go throughout the month, or start and end quickly. There is no way to know when this will happen ahead of time because it depends on the weather and temperatures. It can also start in early August and be over by first weeks of September. But in general, you cannot go at the end of September, that's often when the first "novello" wines are out so it means the harvest is over by then. SO I'd recommend earlier in the month if you can.
2.
participating in the harvest: simply put, you can't participate! That's why you aren't fining info online about it. For safety reasons, only hired people on payroll covered by insurance can participate. You can certainly stay at a winery in the right moment and be surrounded by the experience of a working farm harvesting but you cannot actually participate or go into the vineyards at that time.
So for these two reasons, I can recommend this: take a look at the various wineries in the area, then contact them first directly and ask when they think the harvest will happen at the end of the summer, telling them you're interested in being there when it happens and only then proceed with booking a stay. It might be earlier than you have planned so it might work with your plans or it might not, since I don't know if you already booked travel. Start from here:
https://www.discovertuscany.com/tuscany-accommodation/vineyards/