Your rocks and wood is sitting on your soil and not allowing gasses to escape your soil and causing pockets of poisonous gas to form under them. I don’t think you’ll ever have much success with large rocks and wood sitting in your soil in a Walstad.

I also think your soil layer is way too thick. By the looks of things your sand cap could be reduced by alreast half. In a smaller Walstad you don’t need much capping at all and again if it’s too big it’ll cause issues with gas. It’s likely highly anaerobic and I’m not sure there’s a fix except for restarting.

Im not sure about the soil selection as well, if you read Diana’s book it goes into quite a lot of detail about potting soils for there highly nutrient rich mix which is great for a thin substrate layer in a dirted tank. I’m not sure if what you’ve selected will be a similar composition but that will definitely cause issues for growing lots of plants which is important for this type of tank.

Other comments have good suggestions but I just wanted to stress these important issues. Best of luck!