I was in desperately in need of a user-friendly add-on that would allow me to easily set the resolution for each camera in the scene independently.
As none of those I found, including both paid and free add-ons, did what I needed in a satisfactory way, I decided to make my own. And here it is:
https://github.com/Gatada/MultiResolutionCameras
I’ll make a video of how it works, as the rendering frame added by the plug-in actually updates in real time, much like the rendering frame of Blender itself.