Thank you very much for your interest!
Thanks to Cassandra for the nice ideas: in this period I have some exams to do but in a few weeks I will make some changes sure! I will inform you on this thread.
For the iOS issue: This app is made with "Thunkable" and it seemed to let me download the app for both Android and iOS. I was wating for an email with the link to download the app on iOS to upload it on google drive, after a day it has arrived, but it's not what I was expecting... it is a link for the developer to test the app on a single iOS device, so I can't publish the iOS version. In order to do that I would need the Pro account that costs 25$/month!!! And it would let me to publish the app on the Play Store, iOS app store and as a web app. Unfortunately I can't do it... sorry. I'm editing the first post right now!