Unless I misunderstood “cloud service functionality”, an Obsidian vault can be placed almost anywhere on the file system. For instance, a remote/WebDAV drive or even the Dropbox/iCloud Drive/Google Drive directory.

Migrating is as easy as moving the vault directory from one location to another, and pointing Obsidian to it.

As always, on iOS, there are some caveats as it lacks a traditional file system. So, the Obsidian app cannot access the vault directory on, say your Dropbox. But there are workarounds for it, like hosting the vault on a remote Git repository - which is what I ended up doing. Of course, this is a non-issue on Android.

Obsidian has a help page that goes in detail about what I just said.

As for the Git repository workaround, I referred to this article to arrive at my current workflow.

As an aside, I would like to touch upon my experience with using the inbuilt sync on apps like Agenda and Joplin - both offering syncing using iCloud and Dropbox while the latter offering a whole lot more. It is a flaky experience at best, wherein a significant number of notes never really sync between the devices. This forces me to use my phone to view a particular note while my computer for another. This is where the plain text file foundation for apps like Obsidian and Logseq wins me over.