Following on from the helpful suggestions, I bagged a used goretex-pro jacket to try, by mountain equipment. Quite surprised how close fitting it is, their small is said to be for a 38"-40" chest and I'm 39", yet I find it snug even just over a t-shirt. However, I'm hoping that might be good for my purpose, since (as I understand it) air gaps can slow the transfer of sweat and also allow parts of the shell to become cold and clammy.
It was wet here today but reasonably cool, so not very demanding conditions to test in. But I stomped up a few hills and it didn't get oppressive or damp inside, so that is encouraging.
I'd also ordered a HH 'lifa active' base-layer to team with it, but there was a stock problem - in fact many things like this seem to be out of stock all over (not sure if it is just me being unseasonal, or if it is pandemic related). So I decided to order an odlo 'active f-dry' base layer instead, which is also a dual material/layer approach to wicking and seems well regarded in fields like cycling and running. (Possibly even more open weave ones would be better, but I'm struggling enough with close fitting, without also looking like fishnets). So high hopes when that arrives, too.