I just use Remind, which is the best calendar app ever. If you aren't happy with the calendar on Thunderbird, do you maybe want to take the step to using a full project management app, like ProjectLibre, GanttProject, Gnome Planner, et. al?
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At one point while looking for a calendar app I needed one that could import events from a text file (not ics) and khal was recommended. I was able to import 250 events in one command. I didn't want a cli calendar, but I ended up just keeping that one installed along with todoman for task lists. It isn't ideal, but I haven't noticed the problem you have said about due dates and start dates.
I just mark the due date as an event with a date using Thunderbird's calendar

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