Vello is progressing well, it's most of what I spend my time on these days. I can't promise a date for a stable version, but I would say that for the most part we're on our roadmap.
I'm also interested in quantitative comparisons with the Rive renderer. My personal feeling is that the compute shader approach is more scalable than pixel local storage, but the latter does seem to perform nicely especially on Apple Silicon GPUs.