Help Black Spot on Rhodedendron

I would appreciate some advice on my rhodedendron, it is covered in black spot. It has been really overcrowded too, so it has grown very spindly shoots out like spider legs (but then I dont know if this is the norm?). I am new to gardening, but my instinct would be to prune it back really hard to the main large stems, as I say most of the leaves have black spot and it is starting to transfer to its neighbours.

thanks for the help