Bridging server side and client side rendering is going to be really tricky, and it may be fairly jarring from a UX perspective. Routing is going to be your biggest challenge, as the SPA will be effectively reinitialized when you route back to it if you keep it as-is.

I would try switching the SPA to render on the server entirely/for specific components if possible, knowing there will be angular code changes to support this, then do a progressive blazor rewrite of individual paths. Your app is going to run like hot garbage during this transition though, so you'll need plenty of buy-in to attempt this.