I've been using boring tech for that:

* Python/Django framework (Django itself has 17+ years "on the road" - that's forever enough for me) * Postgres as my main database * Bootstrap as main UI (customized via Sass - not too fancy though) * Unpoly (https://unpoly.com) for frontend and the SPA "feeling" of the app - absolutely no JS frameworks (React, Vue, etc.) - my SaaS product will be mainly CRUD operations, no fancy/dynamic stuff.

Change Django for Rails (or any other "magic" framework) and you're good to go.