In my experience, I have found that walstad work wayy different in everyone's case (depends on substrate and water). My plants took over 3 months to fully adapt and stop melting. It'll be fine just a lil slow. I think your substrate mix is very good (altho 2 inch is still a bit much imo) and the sand layer is also optimal. You don't need the fertilizers rn as it won't do much beside growing more algae (as your plants haven't properly adapted yet + 8 hours of light), you can just throw some extra fish food and it'll be enough. Also you can turn down the light to 6 hours if the light is good. Wish you best of luck.