Non-leaking suit - Check. Diving dry is always a Whack-a-Hole, but I fix every leak - no matter how small - before the next dive. That is why I carry a spare dry suit with me.This is just my opinion, non leaking suit first, wool base and good undergarments are much more important than a heated vest, I have and sometimes use the vest, the last version of the venture.
the battery placement will affect trim, my batteries weight 3 lbs and sit longways just below my rib cage, adjust as needed.
I prefer to using enough undergarment and lead to dive without using the vest, I personally will mostly feel the cold on my upper shoulders and upper arms, the vest doesn’t help that much but does take the edge off.
undergarments do lose some insulation over time, I would lean toward the halo 3D or the newer AR (?) but that’s just me, when my vest dies I wont replace it.
Wicking baselayer - Check. I've always used synthetic since I react to wool. Maybe the newer wool won't do that, might be worth a try.
Undergarments losing insulation over time - Check. The 4th Element Arctic 2-piece has less than 20 dives on it. So maybe I need a warmer set (Halo 3D or BZ400), but I'm afraid of needing more ballast.
I just wish I had a test body of cold water close by so I could actually do some experimenting.