Downsides with Glitch compared to your wish list: it's not self-hosted, and not really WYSIWYG, though your Glitch project is deployed and redeployed instantly so you can see it side-by-side. Collaboration between multiple people at once is very well supported, though.
As for the site generator, 11ty (aka eleventy) is my favorite one to use on Glitch. (My preferred starter project is a version of 11ty-base-blog modified to run as a production site on Glitch. Jekyll or Gatsby are possible to get running on Glitch, but you'll struggle more.
Maybe Glitch? There's a way to connect your Glitch projects to deploy to a more production-type site with Firebase if you need it to have 100% uptime and no loading screen when it hasn't been used for a while.
Downsides with Glitch compared to your wish list: it's not self-hosted, and not really WYSIWYG, though your Glitch project is deployed and redeployed instantly so you can see it side-by-side. Collaboration between multiple people at once is very well supported, though.
As for the site generator, 11ty (aka eleventy) is my favorite one to use on Glitch. (My preferred starter project is a version of 11ty-base-blog modified to run as a production site on Glitch. Jekyll or Gatsby are possible to get running on Glitch, but you'll struggle more.
Thanks, I'm giving 11ty a try!
Question about Glitch and the site generation: do you know which conventions should be followed to do pre-processing in general?