Salute!
I am surprised nobody has mentioned monitorng the "single pilot" as well as the plane. I realize the SLF may not be aviators and would be fairly helpless if the pilot was suddenly incapacitated, but several fairly cheap devices are available if the pilot is dozing off or still can communicate.
The ring I wear for a breathing disorder (COPD) got rave reviews from private pilots flying out west in high terrain. It uses bluetooth to talk with my iPhone on its continuous mode if I wish, but I get alerts when it vibrates fairly strong with or without continuous comm. The pilot reviewers say it tells them when they need supplemental oxygen if their oxygen saturation gets to a specific level, the FAA says - 92% or so, if I remember. The thing also also monitors pulse rate and records for up to 10 hours so you can reconstruct your history later.
Gums sends...