Have you thought about using XData, Aurelius, and Sphinx to build something similar (a common REST API) that basically bolts onto whatever DB that Aurelius supports? And it could run in whatever environments that Delphi and those tools support.
It just seems very odd to me that a vendor of advanced Delphi tools would be using .NET to build their next-generation version of myCloudData rather than Delphi and their own tools.
What I'm looking for is something similar to Memberful, but that lets me add tables and fields via the REST API that can, but aren't required, to be exposed via the UI. Memberful doesn't let you change the DB at all, and it gives you 5 fields in the User's profile that you can name whatever you like and access via the API -- but they're just strings and the user can access them and change them via their UI (they can't be hidden). So you can't save BLOBs or binary data or JSON or any user-related state data that the user doesn't need to see.
So to use Memberful, I'll need to maintain a separate DB to save member-specific data that can sync with what's in the Memberful DB but cannot be accessed from it. In other words, I'll need two DBs instead of one because Memberful has no extensibility.
The nice thing about Memberful is that it handles ALL of the membership details: signups, one-time and recurring payments, authentication, member levels, etc., and all of those details are available on a per-member basis via their API.
AirTable is similar except it's designed from the ground-up to be a friendly drag-n-drop UI-based app builder that uses DB tables underneath. The API lets you see into the DB, but that's about it (from what I can tell).
At the other extreme is your basic MySQL DB. It supposedly has a REST API that you can activate, but it requires root access to set up, and you have to run it in a dedicated server because of how it grabs the incoming traffic from Apache. There's no special UI, it does not handle payments or know anything about any kind of particular usage model -- you need to build whatever you need on top of it. But there are dozens of high-level design tools that work with it.
I also work with a tool called AppGini to build web apps; it spits out php code and uses MySQL, but they don't have any sort of REST API. Anything you add to support REST needs to be built by hand. I've found several tools that will create a REST API that mirrors the DB, which isn't usually what you want -- the REST API should implement Views that restrict access to certain things for security and data integrity purposes. Which gets us back to using XData, Aurelius and Sphinx to build a higher-level API tailored for a specific purpose like a membership site. (AppGini will let you build a fairly simple membership site, but it doesn't handle payments.)