I’ve also got MT500s and some of the £40 decathlon mtb trousers (not those overtrousers). As above, MT500s in proper shitty weather, but also just in the cold. The decathlon ones are a different thing – lighter, not Waterproofs. The MT500s are a fairly heavyweight waterproof. As is the corresponding jacket.
I like my MT500 trousers and jacket for what they are.
I also have “MT500 Spray” shorts both new and a couple of years old. They’re a much lighter sort of thing, with decent waterproofing for something that’s not really aimed at winter. The old ones erring more on the waterproofing side, the new ones more on the light shorts side. They do trousers with the same name, although I’ve not seen them in the flesh.
This time of year I alternate MT500 trousers with these shorts. Shorts just enough to keep me dry if it’s not actually raining.