QGC on HereLink V1.1 - shoulder button won't do "Trigger Camera"

Can you explain the thought process behind setting RC7_OPTION = 9 and RC7_TRIM = 300?

Because what you have done with that is to define RC7 as camera trigger with the trim (which is irrelevant for a camera trigger anyway) about 70% out of range (for a typical range of 1000 to 2000).

Try to set RC7_OPTION = 300, as explained here: