I also make blackberry port annually. I add ~10% red raspberries since I have them on hand. Also raisins, light malt. I don’t have access to black currants on my property but if I did I would ferment them separate until I knew the 50/50 blend is what I wanted.
You won’t need, nor want, the sorbate. Fortifying to 20% alcohol is all you need to stabilize the port. I do not fortify with any spirit other than Everclear, it’s tasteless and it’s so high in alcohol you don’t need much, which preserves the flavor of your hard work.