If you've stayed in Castellina and in Chianti already, then why not go further south and stay closer to Montalcino and the Brunello wineries there if you'd love to visit the wineries there? Actually, any place between Siena and Montalcino or even closer to San Gimignano to the west of Siena would be good bases, particularly if you want to visit both Chianti and the Val d'Orcia (as well as Siena).
Take a look by area, then by size:
San Gimignano area -
http://www.tuscanyaccommodation.com/san-gimignano/
Montalcino -
http://www.tuscanyaccommodation.com/montalcino/
You can also guide your decision on the area of where to stay by also first deciding you want to stay on a wine estate or not - take a look at some of these:
http://www.tuscanyaccommodation.com/wine-tastings/
Train to Florence from Bologna is a good idea, then rental car once you're ready to leave Florence so you have the ease of moving around on your own schedule.
A note on the balsamic vinegar - that's an IGP protected name and method of production in Modena, so in Chianti and Tuscany you can find estates making vinegar who cannot call it balsamic vinegar at all without "passing through" Modena. If they make balsamic vinegar, they need to send their grapes to Modena and have the vinegar produced there... or buy it from there and re-brand it with their name to sell as their own. I doubt they can produce it in Chianti at all, at least not call it that.
Tosciano near SG is one of these I found with balsamic vinegar on their lists, but they are big enough to have various estates so it could be they are also in Modena:
https://www.torciano.com/en/wine/balsamic-vinegar/
Volpaia near Radda in Chianti is another one, making vinegar and not claiming it as balsamic -
https://www.castellodivolpaia.com/en/borgo-di-volpaia/acetificio/
This one outside Panzano and also has balsamic vinegar under its own label but considering it specializes in olive oil, not sure where the vinegar is produced -
https://www.oleariadelchianti.com/it/company
https://www.oleariadelchianti.com/i...ati/aceto-balsamico---quadra-0,25-l?idprod=87
As for Brunello and wanting a top quality producer: if they have the Brunello DOC seal, it is likely to be good quality wine, I don't think they'd put it on the market otherwise. Small wineries here might only make a small amount and "only" sell locally, not having the resources to market internationally. Because you find a good Brunello overseas, consider the size the company has to be in order to have the resources to be there..... so my recommendation is to do lots of tasting and exploring with small wineries as you'll likely find a favorite that you will not find back home, only here. It might mean you will have to order direct to have more wines in the future, but definitely go for smaller wineries over the "name" or "top" brands.