phil-p
I sometimes use my calendar and tasks app similarly. I use Nextcloud, sync with DAVx5, Etar for calendar, and Tasks.org for reminders. If you don't need/want your calendar or tasks to sync to the cloud, just using Etar and Tasks.org should work the same offline.
Etar doesn't seem to allow something like repeating on "every second Saturday of the month" but Tasks.org does.