It is not that simple, sadly.
While many iOS cloud file providers allow you to add files from their storage to other apps, almost none allow you to add or use folders. The only iOS file provider that makes full use of iOS APIs for this purpose is Working Copy, as far as I know.
I don’t have experience with the services you describe, but I tested Synology Drive just now and my suspicions are correct. I imagine it’s the same for the others.
So, sadly, until other cloud storage providers properly use iOS File Provider APIs, there’s not a lot Obsidian’s devs can do to make this work.
Or, well, technically, I think the devs could put in a bunch of work for each and every sync service to support them. But this isn’t really fair. Synology and Dropbox (and so on) should put in the development work so that all other apps can use the features you’re looking for.
That said, Obsidian’s devs have said they’re looking into what they can do to make vault storage more flexible.