Thunderbird application settings being client side, have a separate sync design, unrelated to thundermail. I don’t remember whether they finished it, but I do remember they were working on a sync feature.

I highly suggest Thundermail (tb.pro) if you wanna switch to a libre email provider. Mozilla hosts stalwart and their own libre mail client for you, along with providing 15 mail aliases and 3 custom domains, for a small price. I have been using it as my daily since past few months. I haven’t found any bugs despite it being in beta.