Looks like you replied to the wrong comment(?)

Regarding the Nextcloud's sync functionality on iOS, though: Does it support two-way sync? And do note-taking apps like Notability support it, too? Last time I checked, my impression had been that none of them does – which is a deal breaker for me, given that I'm a heavy user of Syncthing and have really gotten used to its instant two-way syncing functionality to keep all my devices up-to-date.

Ah yes I did reply to the wrong comment. And support is kind of universal but not deeply integrated in apps. Nextcloud adds itself as a location to the iOS Files app so anything that has the ability to save to Files can save to Nextcloud.

The problem is this is more of an import/export workflow rather than seamless syncing. I use it a lot while drawing, the files are all stored internally to the app but when I’m done I save the result to Nextcloud.

It would be nice to have apps automatically save everything there but I think Apple wants to avoid having these remote storage setups as primary storage so your device is still usable with no network connection.