Which version of Thunderbird are you running?
Iām synching calendar and contacts with Thunderbird as well and I use the Addons:
- TbSync - https://addons.thunderbird.net/en-US/thunderbird/addon/tbsync/
- CalDav & Cardav (Provider for TbSync) - https://addons.thunderbird.net/en-US/thunderbird/addon/dav-4-tbsync/
These work perfectly for me and got a rework for the last big changes in Thunderbird 68, which seemed to start handling Addons differently.