"" I also might switch from boots to trail-runners if I knew how to stabilize my weak left ankle better. I sprained it very badly in my early 20s and it re-sprains about once a summer even in boots.""
A few years ago I sprained my right ankle so badly that it took a full year to heal. The doctor told me while examining me that I'd have been better off to break it. I told him, "I saw the root, as I was stepping down. I saw it. I don't know why my ankle rolled- I must have weak ankles." He told me that "weak ankles" are a myth. It's not that we have weak ankles. It's that we walk 99% of the time on hard, flat surfaces and the speed of the connection between our brain and our ankle gets slower and lazier because it's not needed. He told me what I needed to do was speed that connection up, by doing balance exercises. He had me balancing on one foot and writing the alphabet with the other. Balancing on one foot, and tapping the other foot in front, to the right, in back, to the left...with my eyes closed. Yoga balance poses are also awesome for this- tree pose, for example- practice doing it normal, then try looking up. That changes everything. More of a challenge is to close your eyes.
I used to swear I needed boots. Now I don't. I hike and backpack in trail runners and have for the past two seasons, not only injury free but more-or-less blister free as well. I was never able to go blister free in boots- they simply keep too much moisture inside them. I'm not particularly lightweight, either. My full loadout is between 25-30 lbs, generally.
And your legs will feel so LIGHT in trail runners! It truly makes a huge difference in your fatigue level at the end of the day, even if you don't do a thing to reduce the weight on your back.