After 3 days without any random reboot I think I can make a summary of my path to stability. As usual with reboots, diagnosing can be frustrating due to multiple causes for misbehavior. Most of the problems were caused by the Facebook app or by old implementations of microG & Xposed. Once I reinstalled the latest frameworks, made sure that everything was working perfectly, and got rid of the Facebook app, the phone became stable.
The Aix Weather app doesn’t appear to cause reboots, differently from what I stated previously. It was Titanium Backup that was causing easily reproducible System UI crashes during scheduled backups overnight, messing up with many apps including Aix Weather. Those System UI crashes appear to be another potential source of reboots, so replacing Titanium with My Backup solved my remaining problems (updated the wiki with this information).
Thanks to all for the wiki info and other suggestions.