[Non-scalable stack]
DB: Google Sheets
API: Sheety
Web: Ionic/Framework7
App: Cordova with above
Built and tested with hundreds of daily users, but will be expensive to scale and probably has to replaced at some point.
[Solo-scalable]
Back end/DB: Parse Server + MongoDB Atlas + Heroku. No API needed.
Web: jQuery
App: whatever you know
Built and sold one company with this stack, and done some low budget contracts with this. A little more expensive and buggy to start with, but easy to scale to over a thousand daily users.