I don't know how usable SpectrumOS is so far – I guess we'd have to compile it ourselves in order to find out. Either way, it is being developed quite actively, see https://spectrum-os.org/git/
As for how it compares to Qubes, I don't think I'll be able to tell you more than https://spectrum-os.org/design.html & friends. I suppose the upshot is:
- KVM instead of Xen
- One VM per application
- Single file system for user data (to which users can grant VMs access on a folder-by-folder basis)
- Package system from NixOS (nixpkgs) for reproducibility & immutability
Also what do you think about Subgraph OS[1]? Although I think it is not maintained anymore, or is it?
[1] https://subgraph.com/img/sgos.png (old image which I remembered it by) (https://web.archive.org/web/20241206072718/https://subgraph....)