I use a Static Site Generator (SSG) regularly.

You will have wysiwyg when you develop locally.

Here's an overview over some tools: http://staticgen.com/

I see, yeah I use Next.js pretty regularly, you mean on hot reload then?

You still need to write your content as Markdown or something else, right? I feel the editors of Substack give some nice features like shortcuts for bold, italics. What do you do about that?

I tried several on this list a few years ago and found Jekyll the most flexible/enjoyable for my use case. It's a bit dated though.