It looks like you are well on your way….Years ago, before my shoulders gave out and I had more free time, I was pretty obsessed with blacksmithing. While attending a forge-filled weekend, Peter Ross and Ken Swartz of colonial Williamsburg demonstrated forging an axe head. I watched with intent.
I know a tommahawk is a little different but they prepped the blank by fullering from the top. They left a mass of iron for the poll in the middle, thinned/ shaped the cheeks, and left a step where the bit would come together in front of the eye. After forge welding the eye shut, the forward part of the bit was left open. They held the head poll down on the top of the anvil and used a square piece of metal on the diagonal to open up two sides of the bit. In other words, the piece of tool steel for the bit edge was not a flat piece (as you used) but a square piece maybe 3/4"x 3/4".
The next step really blew my mind- They brought the two pieces out of the fire at welding heat and tapped them together with the head upright on the anvil. The light taps required to weld the two didn't even deform the cheeks- it stuck and back into the fire it went. After another heat, it came out of the fire and the finished forming it flat on the anvil. This made the weld in the middle of the bit first and gave plenty of thickness of tool steel to be forged out afterwards. No void at the weld.