If you're looking for a different approach, I moved from Nextcloud to Radicale for my family calendars, which includes ToDo functionality.
From an app point, for Android I'm using Fossify Calendar (which I think you're using?) and Tasks.Org ToDo - and this definitely handles recurring tasks (inc. with different types of schedules)
From a remote access point of view, I have HA Proxy to convert the internal HTTP traffic into external HTTPS traffic (with Lets Encrypt certificate)
(Yes, I also have a VPN for other things... just focusing here for the calendar / todo)
You've reminded me of a slightly off-topic point...
I tried to put Linux on an old laptop for a friend so their kids could use it... it had some weird (Realtek?) chip that was a combination of things (ie video and networking?) and Linux just couldn't drive it, so I had to give up.
That's the only Linux failure I've had and it was also the one where I told them it would definitely work...