Ultimately I ended up with the same solution. The keyboard tray under the desk made a huge difference. You have to sit as if you're riding on horseback to really make that work.
Well then, we have JUST the thing for you here in Japan, a rodeo dieting machine!
https://images.globalindustrial.com/images/275x275/NOS_B1691...
Notice the height and the circular foot bar around the base. Hooking your feet underneath the seat pushes your center of gravity under your trunk, allowing you to sit in a balanced, relaxed posture with a straight back at the edge of your seat (important to prevent your legs from falling asleep).
Edit: oh, also I use the classic Microsoft ergonomic keyboard.
My elbows nearly reach my hips and I cannot use chairs with arms or desks that are "normal" height.