@LesserBlackDog - I hear you RE removability, and am trying the cardigan route as a first step.
Still like the idea of a very warming base layer though. Would be ideal if I could just wear that and my button ups around the office.

As for a down vest, I have coworkers that go that route. While I don't think it looks bad per-se, I can't seem to pull off vests. I've tried, and somehow they always seem to look just... odd on me. For whatever reason, I'm just not a fan. So while good advice, I'll probably skip that path.

The sweater you linked from Gap is the one I have. I have it in several colors. My house is always cool and I can't keep it on in the house. It's just too warm.
That sounds like my kind of sweater! If the one I ordered from Old Navy doesn't work out (or more so if it DOES work out), I have the second cardigan I'll order on lock. Thanks for the real-world testimony there!

The fleece lined base layers provide a LOT of warmth. My wife also runs cold and she was toasty with them skiing.
If they're that heavy, I should probably go with something medium-weight. To @LesserBlackDog's point, I don't want to overheat and have to try to take off everything in the bathroom.

Could go with plain old cotton. Long sleeve waffle knit T and long john bottoms
- I have some of these, and wear them. The basic cotton waffle knit isn't quite warm enough. I find I reach for my coat often enough to justify upping the ante a bit. I don't really need the long john pants because many of my pants are built to be extra warm (uniqulo heattech)

A lot of people here have said good things about the Uniqlo Heat Tech long johns and they are affordable
given my positive experience with uniqulo's heat tech in pants etc, I was considering buying a heattech undershirt to see if that makes a difference. With your comment adding support, I'll pull the trigger on this to try it out

@Burner thanks for the ideas. Gap cardigan is definitely on my shortlist. I do already own the 32 degrees base layers from costco, and they don't offer much warmth. Will check out the peak velocity option from amazon though! Bottom half is usually ok, since I frequently uniqlo's heattech line in chinos.