My question is: can I achieve a similar result (I need an Insert page where I insert customers, a modify page and a report page) simply using AppInventor? Is TinyDB enough for this?
yes, that's easily possible, to start with App Inventor, do the tutorials to get to know how everything works
also helpful in your case is to learn, how lists work in App Inventor, more info see here
there is also a very helpful App Inventor forum in case you get stuck