If you are relying on SEO to gain initial traction, your project is going to fail. Start worrying about SEO when you have a core base of users who are actually using your product and by then you'll be more interested in refining features for your userbase that you won't fall into the feng shui trap that is SEO.

edited to add: The reason I discount SEO like this is because SEO is essentially chasing a search result against competitors who likely have more budget, more resources, and more time to play that game. Spend your time on making your product better, aggressively market directly to people instead of relying on the passive results of SEO, and by the time your project takes off, you will hopefully be in a spot where you won't care very much about why using react-helmet doesn't help with Facebook shares.

Nobody mentioned initial traction or if it is too early to worry about SEO.

As I asked in my question, I also mentioned the need for og meta tags for social sharing, which is something that is often important for clients I talk to.

The appeal of a static SPA hosted on s3 is great, but I have trouble getting good responses about such fundamentals from people who advocate this architecture.

Google will load JavaScript but you can also use react-static to do static SSR if you’d like your first render to be fast or you’d like better SEO from crawlers that don’t read JS.

Ping me if you’d like to know more.

my question is directly related with parent's answer, which, afaik, react assets on s3 + lambda won't allow SSR.

You can do the first render statically into s3 and then attach react. So basically your build does the SSR. It works if you have a smallish number of pages.