G'day K Dog
You are right with armpit to shoulder height for booms.
Work on balancing your harnesses lines first before tackling other problems.

Balancing Harness lines, Hope this helps
In either beach or water start position, hold your sail as though you are sheeted in, & position your hands so that there is equal pull on each arm.
Take note of your hand positions.
Move your harness lines to the mid point of your hand positions.
Go for a blast and fine tune the harness line positions (usually 1-2cm).

Pull on back hand / spin outs = move harness line back
Pull on front hand = move harness line forward.
PS Adjust your harness lines before every session.
PSS On your video take note of your hand positions vs your harness line positions.
PSSS Hopefully your local spot isn't sheltered at the shore, as it is easier to balance your harness lines on the beach, then in the water.
Squish