If it’s just for dog walking in horrendous rain, then I would look at milking parlour tops. Mine is waterproof enough to scrub with iodine and a hose while still wearing it. They aren’t the most breathable, but very waterproof for reasonable price. The brand I have is called monsoon.
otherwise I would be looking at Musto brand, and other hiking/sailing brands for truly waterproof.
Regatta. My cousin is a regular Lake District Walker, gone all day in all weathers type. He says he’s been through lots of brands and says it’s pointless paying hundreds when a bog standard brand does the same job.
I have Mountain Equipment coats and I find them very good, although expensive.
A professional dog walking friend of mine just bought a fisherman jacket and said it’s brilliantly waterproof. Not expensive either (£22). I think she bought it from Amazon - just checked and it’s called Fortex Airflex. Not very flattering but who cares if it keeps you dry.
My hubby who is a shepherd so works out in all weathers buys his coats from an Army Surplus Store and says they’re better than anything else.