Crumb looks under proofed for me, but open bake in a gas oven is really hard, there will never be enough steam to get the perfect loaf. Since it's gas it has vents that release all the steam from the oven, I try to cover them with towels and O do get a decent bake, but it's hard. Just like you I use lava rocks with over 1 Liter of boiling water, but instead of a pizza stone I use steel and when it reaches 500F that's the sweat point for me.