This is an interesting subject to me. Over the last 2 decades, I have made 18 experimental 20” wheel bikes for myself, along with at least 2 dozen forks. They are more like “take-apart” bikes that do not dictate limits on head angles or fork rakes. I saw no point in trying different head angles, and stuck to 70 or 71 degrees for my “all-road” sort of riding. But fork rake (with the resulting trail numbers) seems to be the strongest determinant of how the bike handles. I tried rakes from 35mm to 13mm. Turns out, after about 100K miles of riding my small wheelers of varying geometries, I like plenty of trail. I am very happy with my most recent bike, a 71 degree head angle, 15mm fork rake, 42mm tire (406 wheel), which yields 70mm trail. I find nothing nervous or squirrelly about it, and it rides easily hands free.

There are good suggestions in this thread to tame a nervous handling small wheeler: add weight on front, bigger tires, longer stem/wider handlebars. Less fork rake is what has worked for me.