Hello, I am faced with the same questions, and for me (for now) answers are next:
Q:Where should I begin reading to learn about app-programming?
A: I am wrote my first app using QT6 book https://www.qt.io/hubfs/_website/QML%20Book/qt6book-with-frontpage.pdf?utm_referrer=https%3A%2F%2Fwww.qt.io%2Fproduct%2Fqt6%2Fqml-book
Q: Which IDE should I use (on Ubuntu 20.04)?
A: Official Sailfish SDK Sailfish SDK | Sailfish OS Documentation