According to the documentation you also need MacOS Tahoe. I have all of the components you describe plus MacOS Tahoe Beta. Calling SystemLanguageModel.default.availability returns .available, but I get the following error:

"There are no underlying assets (neither atomic instance nor asset roots) for consistency token for asset set com.apple.modelcatalog"

Not sure this helps, but running MacOS 26 (beta) seems to be a step in the right direction.