Sounds like you want an overlayer, rather than something pile and pertex
The former keeps you warm when you stop. The latter is for active use and is very breathable, but typically needs to be cold otherwise you sweat buckets
Something with synthetic insulation (primaloft for example) would be perfect.
Next decision is arms or no arms (pack size and warmth)
The Montane Prism is very popular on STW. I don’t personally have one as I generally prefer stuff without a hood so I can wear a separate hat.
My Montane Fireball Smock is the smallest, lightest, full length sleeved, synthetic top I have. Won’t necessarily suit all your needs but its a good candidate