What I was taught, growing up on a cattle ranch and spending way too many hours a day in a western saddle, was the stirrups should let you stand in the saddle with about 3 fingers worth of space between your pelvic bones and the saddle seat.
I learned to measure before I got on by putting the middle finger of the same leg (right hand, right leg) on that button or concha that’s below the saddle horn in front of the fender and putting the stirrup up into my armpit. The bottom of the stirrup should hit just in front of the armpit, maybe even a skotch (is that a word?) shorter.
I do hope that makes sense. 
Find a good saddle maker to help you adjust, or get a knowledgeable western trainer to take a look.
Also, what FEELS right to you? You should be comfortable, not tearing up your knees with too short stirrups or feeling like you’re always chasing too long stirrups.
In the saddle, drop the stirrups. My good length in a western saddle is when the bottom of the stirrup hits just above my ankle bone.
My 2 westerns are both leather, one very old Montana Saddlery and one newer Billy Cook, have had to have holes punched between the factory holes to fit me.
Also, don’t assume both stirrups should be the same length. Mine are not, due to long term issues with one leg, but not everybody’s legs are the same length and it makes a difference.
BTW, beautiful photos! Thanks for sharing and good luck.