If super productivity + nextcloud ends up not being a good fit, i'll give this a try, thanks :)

I'm using a similar stack, Radicale on backend, DAVx5, and Calengoo as app.

Calengoo is not open source but you can buy it directly from the developer without Google integration. It's the only calendar app I've found that also does tasks properly, in that if I dismiss a task without explicitly marking it completed the notification will come back. It looks a bit strange out of the box but it has multiple themes and it's super configurable.

The one thing I don't like is that it uses a direct connection to CalDAV to mark tasks completed, instead of going through DAVx5 (not sure if it's a limitation in the Android chain or what).

yeah i do exactly this and it's very stable and unproblematic. has made a significant difference in my life