I still prefer Thunderbird Lightning for my calendars (and also not being tied to iCloud ones).
Could you add support for integration with local Thunderbird Lightning calendars (without requiring an unsyncable importing or publishing step)?
It’s unlikely we’ll support any calendar system at this point that isn’t using the system calendar under the hood, i.e. even if you use a different calendar app as your default, if you would open the system Calendar app the events should show there as well. If they don’t, Agenda won’t see them.