The only other thing I've seen in this space is DodgerCMS. It may meet your needs as it seems to do a lot more than Tumbless already. I think I prefer the way it handles credentials too(saved in local storage iirc rather than in a "hidden" file that's technically available to everyone)

https://github.com/ChrisZieba/dodgercms

Regardless, I think this is something that would be awesome to have. You can even throw in some AWS Lambda if you want to move some stuff out of the browser(like generating thumbnails/index pages/etc). Or just use Lambda to run a complete static site generator(like jekyll or pelican) every time the content in an S3 bucket changes.

Note: I've never actually used either of these, but I did spend a fair amount of time researching this type of stuff.