Woohoo Phil!! You must be super excited about registering for this event, my husband is a cyclist and had looked into it but preferring solo and peaceful travel along roads and trails, said this sort of event is not for him

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I am assuming you're both experienced cyclists and in decent shape. While the
official site says it is for everyone, there is a lot of elevation difference over the 400+km of the route (longer route). There is a lot of info given out only to those who register, did you already get the entire pack of info with both routes outlined?
I am also reading on the website that there is a "classic" package and a "no stress" one, which one did you get? What date and departure did you choose, and did you choose the long one or the short route, which I understand is not binding but which will impact where you stay at?
If I was planning this experience, I'd first decide on how many km you want to/plan on traveling each day or in how many days you want to complete it. It seems you have 6 days to complete the event, with a majority of people taking 4 days. It seems they've also added "detours" along the long route, so you have to factor in whether you want to do those.
This summer we were in some towns that are passed by the Tuscany Trail and some locals complained about not knowing about the event and just feeling overrun and not prepared to host the large number of cyclists on those days but I'm seeing some changes as far as organization that likely have been made to address those problems: the planned departures to morning and afternoon over 3 days and "base camps". The first one should help in not having everyone trying to stay in one town on a specific day, and the second in offering a safe location for cyclists who want to sleep outdoors and have fun meeting other people doing the event.
Once you know the km per day or total days you want to take, I think you could easily identify the towns along the way that more or less fall into natural stops along the way. If you have info on where the planned "base camps" are going to be, you can actually use that info to know which sites/towns to AVOID, if that is what you want to do!!
Also, have you already figured out how to get to Campiglia Marittima for departure? Will you be driving there or taking train? Are you arriving from Europe or further away? Will you be bringing your own bike or need to rent one?
As far as luggage services are concerned, I found a few that offer this service since they offer bike tours so are used to offering it to their clients.
https://tuscany.tours/tour/luggage-transfer-tuscany/ and
https://www.dfbike.it/en/richiedi-una-guida-o-il-transfer-bagagli-2/ (based in Siena)
These two also offer luggage transfer service, as well as bike rentals, in case you need that too.
Different types of bicycles are available, from walking (Slow bike), off-road (Mountain bike), to racing (Race bike), including the vintage one (Vintage bike)
tuscanybiking.it
We offer all the services for your bike tour in Tuscany. Anima Toscana it's a travel agency and bike tour operator and we organize cycling tours and vacations in Tuscany. We offer guided fully supported bike tour, self-guided bike tours, one day bike tours and bike rental service. The guided...
www.bike-tour-tuscany.com
If you let me know whether you already had an idea of the days or amount of Km you want to do per day, then from there I can help you figure out the towns/locations at which you could look into for accommodation. I see that many bigger towns are passed (Volterra, San Gimignano, Siena) which means more lodging choices there but it might not match with the km you want to do per day, so that needs to be figured out so that you can look into accommodation in those specific areas.
Look forward to hearing back!