But if I have it right, I have no idea why a JS backend would be required to do SSR; this is exactly the same domain of work php, rails, django have always done, without node.js needing to enter. For an SPA (which I believe is just a bunch of js fetching data from JSON apis and rendering client-side), you'd need JS to handle client-side rendering, but if you can replace that with WASM (when it has DOM support)... I don't see why you'd need JS anywhere
FYI I'm defining everything because I'm pretty sure there's a mistake somewhere in my understanding of the problem