a thick suit clearly reduces this cold impact, but for me the problem with a thick wetsuit in conditions of 20degree air is that i am going to overheat when i don't get wet on a regular bases. also a thick suit isn't usually very "stretch" so it will not be very comfortable to paddle longer time.
i would recommend paddling in "thermal swimsuit" as you said, and from time to time sprinkle a bit of the "cold" water over your neck/feet/head so it won't feel like a shock.
but i also might also not be a measure as for me in switzerland these conditions seem like summertime

P.S: what thicknesses of wetsuits are we talking about? 2mm? 5mm? if you want a wetsuit for your conditions i would recommend 2 or 3mm maximum, gives a bit less protection, but leaves you more freedom to move and heats less.