Thunderbird is currently in the testing phase for Thunderbird Pro, based on the relatively new JMAP protocol, also allowing a more streamlined experience, especially regarding the somewhat limited calendar function via CalDAV (calendar invites only arrive as ICS files via email, for example). I'm aware that there aren't many providers yet supporting JMAP, but for me, that's one of the major issues that keeps me partially hooked to the Microsoft ecosystem.

JMAP Project Website: https://jmap.io/