What about access to shared calendars via EAS?

Access to shared calendars is not part of the standard EAS protocol. Most of the time they are simply not announced via EAS, and if TbSync is not able to see them, it is not able to synchronize them.

It has been observed, that some 3rd party servers do support shared calendars vie EAS, but most Microsoft servers don't. However, Microsoft is working on this as well: https://support.office.com/en-us/article/calendar-sharing-in-office-365-b576ecc3-0945-4d75-85f1-5efafb8a37b4

Besides the Exchange ActiveSync protocol (EAS), some providers also support the Exchange WebServices (EWS) protocol, which has shared calendar support by default. There are also some Thunderbird Add-Ons supporting EWS (ExQuilla, Exchange EWS Provider)