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Really? That's really difficult. All I'd say is try minimise the layers your wearing to keep your temperature down. I'll often ridden in the rain in a rain jacket and basepayer. This also then has the benefit of keeping as many clothes as possible dry for later.
If it's day rides or not exposed conditions, I'd properly ride in a softshell (like Infinitum). I often do use a softshell but I can either return home easily or look after myself if I get cold.
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I wear very little, but still carry the kit and then put it on when I need it, like descents or stops.
My missus thinks I'm a mental but I'm normally running hot when riding so I can deal with most descents without even bothering to put on the layers - I use the downhills to dump heat. Depends on terrain and conditions, obviously but climbing in sub-zero I could be in summer kit for all the heat I generate.
Get a proper rain jacket. No one wants to be in the mountains with less, you could be hypothermic.
You just ajust your riding in the rain so you don't sweat.