Also curious to others’ usage here. Also coming from thunderbird, which has grown rather sluggish with my 2 Gmail accounts, and 7 Nextcloud calendars. Task integration is OK, but some things don’t work, like marking a recurring task complete (I know that Nextcloud doesn’t formally support recurring tasks, but they kind of work). And, now it’s 2023 
Off-topic for the calendar discussion, but try using dedicated task apps. I do not have a particular client to recommend.
Generally speaking, BusyCal for Mac is a superb client. That’s all I know. Not sure what to use on Windows or mobile.
Umm…no…? I was asking about calendars - how is that off topic? I was mentioning why I don’t care for Thunderbird as an all-in-one calendar/task solution.