> how do I get started writing my app to target the Librem 5?
For commandline applications all you need is to develop for GNU/Linux. It should work pretty fine on Librem 5 too. If you want to develop GUI applications see libhandy[0], a GTK+ widget library developed by Purism for Librem 5.
Btw, I wonder : I intend to use KDE on librem 5 (at least eventually, if it's not ready on phone release). Do you know if mixing GTK and QT apps on the system will be as easy as on desktop?