Here are few projects to get started: https://github.com/apptension/saas-boilerplate?tab=readme-ov... https://github.com/ixartz/SaaS-Boilerplate

For the UI, MUI is a huge time saver. It's open-core and thoroughly excellent: https://mui.com/

They also have a lot of pre-built dashboards that tie into various cloud vendors (typically not FOSS though).

https://news.ycombinator.com/item?id=39192304 - https://github.com/wasp-lang/open-saas

SaaS starter kit, MIT license - https://github.com/makeitcount/saas-express-starter-kit

I've used sailsjs multiple times and once I'm up to speed with its concepts, the dev time is really really fast . It doesn't fit some of your parameters

For backend this might help https://feathersjs.com. I’m not affiliated with them, just a happy user.

https://www.saaspegasus.com/

you can have a look at ML, many of my preferred Saas boilerplates and starter kits here: https://microlaunch.net/category/starter-kits