Can t we SEO Mern Stack Applications and reactjs Applications

Question

I created a MERN Stack Application which uses reactjs as the frontend, Nodejs with express as the Backend, and MongoDB as the Database. I want to do SEO for this application but in many articles and resources, I have seen them mentioning that we can't SEO Reactjs applications and instead we need to implement SSR(Server side rendering).

1. My first question is Can we upgrade the existing MERN application to NextJs?
2. any other methods to SEO this MERN Application?
3. Do I need to stop using React as the frontend in my future projects? Instead, do I need to use Nextjs definitely in my future projects?