Anytime you rewrite 12 existing, working web apps in something new it is going to be a huge pain. The only sane way to approach this is the strangler pattern, picking one page/view in one app to rewrite, then another, hooking them up with some kind of client-side router, and slowly letting the SPA consume the old SSR pages. That way you can measure the cost/benefit of each page before deciding it should be added into the SPA.