Assume the following:
- Can be picked up by almost anyone (assume basic HTM/CSS/JS understanding), doesn't require any deep CS understanding.
-Has full-stack capabilities, not just front-end
- Is open source and can be deployed anywhere
- Is complex enough to rule our no code solutions like Bubble or Webflow
While they won't give you an "advanced" front-end out of the box, the majority of functionality you'd need for an SaaS can be perfectly served by standard HTML and forms, for which those frameworks have excellent support (built-in form validation support, sessions, etc).
I am not aware of any complete full-stack framework that combines the functionality of these backend frameworks with a front-end component. Meteor could be an option but I haven't heard of any big project using it and there's probably a valid reason why.
There are lots of solutions that fit your criteria, including this one.
The problem is that none of them will be quick. The surface area for non-trivial app development is massive.
That said, a single-language, full-stack option that uses TypeScript would be the quickest to learn (mostly because of compiler rules serving as guard rails and because it's only one language).
I have no idea what the state of full-stack JS is right now because I'd never use a full-stack framework (or any framework).