I run a decently established website (20k monthly users, already indexed by Googlebot, etc) that has been built on Django a year and a half ago. The vast majority of traffic on my site comes from the SEO that has been built up over the last year.

Recently, the codebase has been becoming harder to maintain, and the hosting bill has been rising on Heroku.

I'm planning to switch to React.js hosted on Firebase, however, I can't justify using a React SSR framework such as Next.js due to scalability. 

My current migration plan is a banner on top of my website that leads to a subdomain of the current site (eg: beta.website.com), in order to get Googlebot to begin using their two-wave crawling process for javascript. Once I am comfortable to release the new site from beta, completely remove the old site and replace it with the new CSR site.

Now, with the knowledge that Googlebot now has a 'two-wave' JS rendering/crawling process, would my website be drastically affected by this change? Would I be better off using an SSR framework, even though my website has already gained some traction through SEO? Is a largescale upgrade like this (entire front-end re-write) enough to degrade the SEO I've built upon?

Thanks!