I recommend using a static site generator. It brings precisely the features you ask for:

  - easy SEO optimization, especially compared to SPA  
  - easy content management, GitOps  
  - fast, scales well  
  - free to host via various services (e.g. GitLab and GitHub)

Personally, I like Hugo. They have an amazing gallery of free templates: https://themes.gohugo.io/

You might enjoy my blog post on how I host my personal website for free with Hugo and GitLab Pages: https://tkainrad.dev/posts/using-hugo-gitlab-pages-and-cloud...

For others interested, here is a list of flat file CMSs: https://github.com/ahadb/flat-file-cms