what happens when you want some interactivity after the initial load? That's why you SSR React. If you used a serverside template then once the page loads, how do you deal with interactivity? jQuery? Plain JavaScript?

Yes why not? If you already decided your site is content-oriented by choosing server-side templating, then interactivity will be limited in scope and easily implemented using either vanilla js or jquery, or even basic HTML forms, links, and CSS menu tricks without js at all. In my experience, while I personally like React when it fits, I've seen too many projects that don't benefit from a MVw approach where React or Vue or another MVw framework seems like a decision by developers to pad their resume, or even by managers to win developers by giving them, like babies, something to play with and show off to their peer group when in reality the app is just a basic LOB application, like the old legacy app it replaces.