Thanks for writing this.

I use Todoist and it syncs quite beautifully with my calendars.
I use Fantastical, (which is Mac/iOS only) but it pulls in all my tasks and populates them.
It’s a 2-way sync, so I can mark them as done in the calendar and they get marked as done in Todoist.
Likewise, I can create tasks in Fantastical, at times to suit me, and they sync straight into Todoist.