My case is a very simple case and I use Google spreadsheets, synced to Firebase's realtime database.

Jamstack static site or @statamic

Our devs who built the Kreg Academy learning platform CMS used word press. Lots of slick plug ins and was super friendly as an end user.

Depends on what "simple", "content", and "management" means to you, but, Jekyll?

Haha, fair. Existing MySQL DB that powers an API, I just want a simple web app to CRUD the data.