You are dealing with two difficult problems here. What you need is a no nonsense product.

For Blackspot you need to use a proven stuff like Triforine. Get the concentrate and dilute it. It's much cheaper this way. And you will need it every year when it's time to get the first flush of leaves and blooms. Blackspot is very strong when the wheather is hot and humid. When you see it, it's too late. So you will expect that after spraying, more leaves will fall before it stabilises. Then the disease will subside and you still nee dto spray monthly to control it.

Rust is very hard to treat. They claim that Mancozeb or triforine will help but I find them equally useless. They will help but will never get rid of the problem. The only way I have found to get rid of rust is to do a strict conrol practice. Whenit's warm, the only way is to remove all infected leaves, spray Triforine and pray. It probably won't help. So wait until Winter, do a hard prune to get rid of all leaves on the plant. Look at the surrounding ground and make sure rust does not affect any of the weeds or plants that could keep their leaves in Winter. If you see, destroy those weeds and plants as well. Rust leaves roses to lay low on some other plants like the weeds, then come back to roses in Spring.