Decrowning is, as you say, just flattening off. I would think the mentioned dimensions would work well too with sweet birch and hackberry for example
I made a silk backed sweet birch board bow last year that came out surprisingly well. Didn't like steam bending though. 64", 1 3/4" wide, 58#/27". Sweet birch is an E US tree though and doesn't extend up into the north country
I've been wondering what the subarctic indians used for bows, so many soft wood trees up there. One think I was wondering about was sizable woody shrubs with tough wood. I've been thinking about Silverberry, Elaeagnus commutata, related to the European Russian Olive, whose range extends frin the Yukon, Manitoba, to Quebec, Minnesota, adn down to Utah. Grows as a week around here in central PA in hedgerows, Has a nice thick (up to 3") stem that is often non crooked or twisted and curved for natural reflex. I have a piece cut, feels pretty heavy, and am going to try it some day when I'm caught up on projects.
Dave