Hi,
In that case, it will be complex to do that.
When you create a manual shipping method via the menu System>Shipping methods, you have a "tax category" setting. So you can select ONE tax category for the shipping method.
Hence, the tax rate that will be use for the shipping method fees will depend on that tax category and not the tax categories of the products in the cart.
So having the shipping method fee taxed at the pro rata of the products in the cart is not possible with only one shipping method for the whole cart with the current system.
The only solution would be to create 2 warehouses, assign the products with a 7% tax rate to one warehouse and the ones with a 19% tax rate to the other one, and then create two sets of shipping methods, one for each warehouse, and then in each shipping method you would select the tax category for the corresponding tax rate.
That way, on the checkout, if you have some products at 7% and some at 19%, the shipping method selection will be split into two shipping methods selections, one for each warehouse, and thus the shipping fees will have the correct tax rate for each group of products.
The downside of that method is that in such case, you have 2 shipping fees added to the cart. If you're shipping methods use a percentage instead of a fixed price, or if they use the "shipping price per product" system then it's not a problem, but for fixed price shipping methods, that means that in such case, the shipping fees will be doubled.