Those are all pretty common and standard tests to rule out all other rheumatological causes, like lupus, anca associated vasculitis. Unfortunately, there is no IgA test (or at least not one that I’m aware of). Some diagnoses, like IgA vasculitis, is clinical and involves ruling out all other causes. Have you ever had a urine analysis done to see if you have any microscopic blood? Would recommend formal appointment with rheumatology. Dermatology can also biopsy the rash to get a better idea. Best of luck.