Struggling with Black Baccara — would appreciate advice

I want to be honest: I'm not getting great results with Black Baccara and I'm not entirely sure why. powdery mildew in humid summers has been a recurring problem despite my attempts to address it. I've been focusing on deadheading spent blooms to encourage reblooming based on everything I've read.

strong disease-resistant root stock is there in glimpses, which tells me the plant has potential in my conditions. I have it in a cottage garden mixed planting with lavender which might not be ideal. hips forming in autumn for wildlife interest showed some improvement. Two stars for now — not the plant's fault, but not a success yet. I'll update if things improve.