I mean generating your HTML on the server. If you use HTML as a semantic markup language, it works pretty well. Then, using Vanilla JS for the interactivity. I doubt it's "too much to maintain"- what is harder in Vanilla JS than Gatsby?

You can write much of your site in raw HTML. For example, your right panel– you can write it in raw HTML, annotate it with some tags. Make it an iframe, just use CSS to get rid of the borders, or an XMLHTTPRequest. As long as you make good use of CSS, you can have a few tags and it'll work.