Ah, okay, I missed that wrinkle.
The resource template API endpoint is looking for data in the structure of the endpoint’s GET output: in particular it’s looking for o:id subkeys on the resorce_template_property and resource_class keys.
The template import/export uses a different process that’s specifically designed for transferring between different installs and therefore doesn’t rely on internal IDs.
This would still be doable as code-only but you’d have to add a step or steps of reading the current properties/classes out of the API to fill in the needed IDs.
We could possibly expose an endpoint or mode in the API itself that accepts the “export” files… I’m not sure off the top of my head how difficult that would be to do.