With the recent change, factorio planner no longer shows how to produce ash which is quite painful when trying to get an idea of what is required

Interesting.
Ash is made in the same way you make Used up uranium fuel.
I bet the factory planer are unable to consider ratios of Used up uranium fuel to and burnt results from pyonodon.
But I don't think this issue is any of this mods concern, I think this suggestion is more suitable for the other mod, ask there if it is possible to consider burnt result types items like the Used up uranium fuel into the planer as some mods uses them like PY and this mod.

Looks like factorio planner doesn't handled burnt results from fuel, and no idea when it will implement that

I think it should be a separate mod. but If you want to make a mod like that you can make a new furnace with a new crafting category "Your_new_crafting_category" add the new crafting category. Also make a function convertFuelValueToFloat(fuelValue)

then you can add new recipes for each item with a burnt_result:

-- Define the function to create recipes for items with burnt results
local function createBurntResultRecipes()
    for _, item in pairs(data.raw["item"]) do
        -- Check if the item has a burnt result and fuel value
        if item.burnt_result and item.fuel_value then
            -- Define the recipe name
            local recipeName = "get-burnt-" .. item.name

            -- Calculate the recipe time based on the fuel value
            local fuelValue = convertFuelValueToFloat(item.fuel_value)
            local recipeTime = fuelValue  -- Assuming 1 fuel value = 1 second

            -- Create the recipe
            data:extend({
                {
                    type = "recipe",
                    name = recipeName,
                    enabled = true,  -- Set to true if you want the recipe to be initially available
                    energy_required = recipeTime,
                    ingredients = {
                        {type = "item", name = item.name, amount = 1}
                    },
                    results = {
                        {type = "item", name = item.burnt_result, amount = 1}
                    },
                    category = "Your_new_crafting_category"  -- Assign to your new crafting category
                }
            })
        end
    end
end

-- Call the function to create recipes for items with burnt results
createBurntResultRecipes()

Seems like this mod need that anyways because of this: https://mods.factorio.com/mod/Seaworld/discussion/65ce726f584c35ef49f64faf