That’s correct, there is currently no OpenXR standardized device driver interface. If you’re interested in helping push the standard forward, you could join Khronos and join the WG. This also gets you in touch with the people from other runtimes who would be interacting with your device: while you can make an extension without being a member, if you’re looking to make a device driver/plugin, you’ll need adoption from other runtimes to be able to run.
That said, there are two options for you: you can of course keep making an OpenVR driver, which should be exposed thru the SteamVR OpenXR runtime. Alternately, for some purposes (works ok for hand tracking, less well for other things that touch the action system more) you might try making an API layer like Ultraleap has done.