I’m about to go out riding on a very damp evening. Thing is, its chilly from the rain, but not that cold. So, if I wear my Fox Sidewinder full-finger gloves, I’ll get wet hands, which will get cold. If I wear my waterproof Endura Stealth gloves, I’ll get hot hands, which will sweat (and its nowhere near cold enough for the Spesh Sub-Zeros).

So can you get a nice thin full-finger glove, like the Fox, but with a waterproof breathable membrane?

No.
Suck it up.

Specialized defroster, very thin with gel seem ace so far, there is a thinner version called equinox….the defrostee are waterproof and have been great so far! Exactly the thread I started a month or so back