Fluffy I only grow english roses from David Austin they are without doubt resistant. Over the years I've tried many and like you got frustrated with black spot especially. After much experimentation, I have finally found a good way of keeping black spot under control, and I use it in other gardens for people who I garden for. I have found it works.
No matter what the rose, remove and burn the affected leaves, even if it means you have just bare stems and no blooms. Clear the ground around the rose, no detritus or dead leaves. Water well and feed. Then spray/soak with fungicide, rose clear is as good as any, and spray the ground around the rose as well. You can then mulch and make sure you have good air movement around the rose. I have never read or seen anyone recommend spraying the ground around the rose, but common sense dictated that the spores that cause black spot fall to the ground and rain falling makes them rise and re infect the rose.
It works for me and my customers, it may seem brutal to strip the rose bare, but they produce new growth within a few weeks, and it's usually healthy.
If you get a further infection just follow the same process. I can honestly say, I haven't had a failure yet.
There are so many beautiful roses, I wish I had room for more, confining yourself to one type is a matter of choice and one I made for more than one reason, but I'd hate to see anyone who can grow whatever is their choice.