It’s true that the original synthetic belay jackets - like the Patagonia DAS parka - had a much heavier weight of insulation, and most modern designs feature lighter weight, but I think in all but the most extreme of conditions this weight may be more appropriate, offering a better balance between weight, bulk and warmth. Worn over a standard layering system - especially the softshell system I described - these lighter parkers will still do their job of keeping you warm when static in normal climbing scenarios (belaying, abseiling, bivvying).
The heavier weight parkers worked better in more extreme situations, such as super cold (Alaskan winter or high altitude mountaineering), or for super-extended belaying. I’ve used the old school DAS parker in the Ruth Gorge in Winter (temps reaching -50), and super long belays in frigid spots, while people like Steve House used his all the way to the top of Nanga Parbat.
So what if you want a system that will do both duties, but will still offer the same degree of warmth and protection?
If you have a specific need (big walling in Baffin for example), I’d recommend getting a bespoke belay jacket from someone like PHD, who could take their existing Zeta jacket and double up the insulation.
A more flexible option would be to have a layered approach to your booster layer, perhaps layering up a hooded midweight synthetic hooded jacket with a micro loft down jacket (or gilet), perhaps even modifying both so they can be linked (velcro, poppers or toggles). With this, you’d wear your basic outer belay jacket for rough and dirty action (wet snow, belaying etc), and save the down layer (stowed in a small dry bag) for bivvying, super long belays or extreme cold action.