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 donāt have experience with the services you describe, but I tested Synology Drive just now and my suspicions are correct. I imagine itās the same for the others.
So, sadly, until other cloud storage providers properly use iOS File Provider APIs, thereās not a lot Obsidianās 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 isnāt really fair. Synology and Dropbox (and so on) should put in the development work so that all other apps can use the features youāre looking for.
That said, Obsidianās devs have said theyāre looking into what they can do to make vault storage more flexible.