I've found that Radicale works very well for DAV calendar, tasks, address book. I combine it with Syncthing for files, and use Obsidian.md as the note-taking application (using the synced folder for storing the notes folder). Of course when you have a synced folder you can use whatever note-taking application/system you want.
I'm not sure if that will work on iOS since iOS doesn't really have a filesystem - there are apps available on iOS for Syncthing and Obsidian, but I don't know if Obsidian will be able to access the Syncthing synced folder. But it works great for me on Linux PCs + Android. Much better than Nextcloud which is too complex and is a pain to administer over time. Syncthing is super simple to set up does file sync very well, much better than Nextcloud does. Radicale is also easy to set up and just works.
iOS only needs to be a client and it has standard CalDAV/CardDAV clients built into the OS. I've used Radicale— it works OK for very basic functionality, but doing something as simple as making a shared calendar involves creating one in the GUI, logging into the server and sym linking them between user directories.
The nice thing about the groupware functionality in Apple Server is that it used all of these standard protocols so it was entirely interoperable with other devices AND it had a nice smooth administration experience.
At the moment, I just pay $15/mo for Cloudron which handles email, is decently smooth for administration though a little more disjointed between the apps than I'd prefer, and can "one click" deploy Radicale, NextCloud, Sogo, et. al. I used to administer servers but it's not what I do now and I have no interested in sinking non-work time into work-like tasks.
I'm not sure if that will work on iOS since iOS doesn't really have a filesystem - there are apps available on iOS for Syncthing and Obsidian, but I don't know if Obsidian will be able to access the Syncthing synced folder. But it works great for me on Linux PCs + Android. Much better than Nextcloud which is too complex and is a pain to administer over time. Syncthing is super simple to set up does file sync very well, much better than Nextcloud does. Radicale is also easy to set up and just works.