Thanks so much @ngomes!
I couldn't get the beta to uninstall the version of 10.8.14 I'd jammed in, but the release version installed and works! I've got a Spacemouse (x86) working in Fusion (x86) all in emulation on a Surface Pro 11 running Windows 11 arm64. Amazing.
Eventually it'll all be arch native, but for now this is a sufficient workaround. Thanks again.