It sounds like we are in similar situations. I have had a rash for over two years. Sometimes mild and other times debilitating. I have been to two allergists and three dermatologist. The allergist said it’s contact dermatitis with my allergy being everything scented, botanicals, dogs, metal etc., basically everything. However as avoiding triggers did nothing. My most recent dermatologist is leaning towards t-cell lymphoma, which as you said, is really rare.