Finally, after a morning trying, I got RPD to find photos on my iPhone. I have no idea why I have been successful this time, but failed to connect to the phone on the previous attempts, even after having read the guidance on this precise topic a number of times. RPD reports 21 images from my iphone, which has in total about 920 photos in ‘My Albums’. Those 21 do not appear to have a common characteristic or property. They range in modification date from October 2017 to 25 December this year. On the phone, photos go back as far as early 2014. I can find no way to get RPD to find the remaining 98%. What am I doing wrong?
Currently Rapid Photo Downloader uses a library called gPhoto2 to access photos and videos on devices like phones. gPhoto2 in turn uses the libraries called PTP2 and MTP to access devcies. Unfortunately the iPhone does not adhere especially closely to the PTP / MTP standard, meaning gPhoto2 has trouble with iOS devices.
As far as I can tell the only solution for Rapid Photo Downloader is to use libimobiledevice on iOS devices, instead of gPhoto2.
It’s a pain in the butt. That’s what happens when developers and companies don’t stick to industry standards! 
I am motivated to fix this problem in Rapid Photo Downloader because I recently purchased a used iPhone — I got tired of my Android phone losing manufacturer software support after only a few years.
This is the single overwhelming issue with Android phones. It caused me to switch when iPhone 4 was announced and my Samsung phone (nice hardware) suddenly became fatally susceptible to all those mentally defective malware creators out there. This situation says so much about the Corporate ethics of Google et al.