In this case, we’re really talking about two separate things:
- Adding external calendars to the Nextcloud server.
- Adding task lists from those calendars to the Tasks.org app.
Regarding the first point, If tasks from those calendars are being displayed, then I suppose that answers my earlier question 
. But creating new tasks or events in those subscribed calendars isn’t possible, neither in Nextcloud itself nor in any connected apps, because of their read-only nature. Also, I’m not even sure if subscribed calendars are exposed through CalDAV to client apps. As I said, I don’t personally use any.
As for the second point — there are two ways to add CalDAV accounts to Tasks.org: either directly through the CalDAV option in the app (which I haven’t tested), or via DAVx5. I use the latter, and it works perfectly, though I only sync my native Nextcloud calendars and task lists with it.
For the simple reason that calendar subscriptions are read-only, Nextcloud is of limited use for this purpose. The iCal subscription feature is really meant for things like public holiday calendars, organizational event schedules, and similar use cases.
Or in other words, Nextcloud is not suitable as an actual hub for external calendars due to the lack of a true CalDAV client mode. So if you really want/need to work with these external calendars, I wouldn’t do it this way and would add them all to DAVx5 on your phone. And yes, you’ll then have to set it up again on a new phone as well as on your desktop and all other devices you might want to use them.
That’s because iOS already has DAVx5 built in, so to speak. Or, to put it more accurately, iOS includes a CalDAV/CardDAV client at the system level, whereas Android does not. DAVx5 essentially fills that gap by providing the missing functionality on Android.