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.
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.
What I'd REALLY love is to be able to run all of my own services in place of iCloud. I realize that this stuff isn't free to develop and I'd happily pay for it. They've been moving further away from standard formats and protocols with no way for others to integrate in the same way and I'm curious if there are any legal anticompetitive actions on the table should they continue to do so.
Before anybody says it— I have business needs that favor using MacOS directly on Apple Hardware and as a result, iOS is a good choice. Moving from Apple would regularly cause me more grief than these things do. No, you don't know my use case better than me and I have exhaustively explored all alternatives both ancient and modern.