Help with senior dog with bad dandruff

I have a 12 year old miniature pincher rescue who has such bad dandruff all over her back and face. She also sheds like crazy, but will NOT let anyone brush her. I'm currently using tropiclean medicated itch relief shampoo and tropiclean shed control conditioner. I've done oatmeal baths but it doesn't help. She isn't a big fan of grooming anyways, but I can still bathe her. I try to bathe her 1-2 times a month to keep up with her hair and dandruff, but I know the breed doesn't need super frequent grooming. She is overweight, but she has been on a diet for 2 years but just can't lose any weight. she has a vet appointment coming up to see if she has any issues that are preventing her from losing weight. she also has patellar luxation and is constantly leaning on me during her bath, so I get soaked as well. She is very specific so I don't trust another person to groom her because she is so specific with who she trusts. She won't even let my mom groom her, and she loves my mom. what do I do?