What I’m trying to do
Sync with NextCloud self hosted server on iOS
Things I have tried
iOS support is not present. I would pay an in app purchase (non subscription) to unlock such a feature.
As far as I understand your question and as far as I’m aware of IT technologies, it’s a limitation imposed by iOS itself : applications can access files on iCloud drive and that’s all. They are totally forbidden to access any other part of the filesystem. So, as long as this limitation stands — and it’s a security related one —, Obsidian will never be able to access files on another sync service like Dropbox, NextCloud, etc. I personally would’ve very much enjoyed being able to sync my notes with a super-secure tool like Tresorit. But, up to a certain point, Obsidian Sync suits me very well.
In the case my affirmations shouldn’t be 100 % accurate, I’d appreciate a programmer correcting me or completing my answer.