If SEO matters beyond a couple landing pages, I find it unlikely that you would be developing a rich web app with that much reactivity. You are more likely content focused, in which case a static site generator or simpler SSR frameworks are easier and fit the use case much better. Even from a performance perspective, why ship the entire react run time if you do not need it?
And on the other hand, if you are developing a rich web app with a lot of interactivity, then do you really need SEO beyond a couple (or one) landing pages? You should develop the web app in React CSR and build the landing pages as static sites to optimize SEO. That is a lot easier to me.