There ain’t no replacement for displacement so if you’re truly glassing for long periods nothing beats lots of high quality down. Synthetic works too but the warmth per fill is IIRC similar to 500-600 FP down and goes downhill with use. If you really want to go down the rabbit hole, read up on CLO value
Rab electron and neutrino are good with solid color options. FL Chamberlin in tan is good too on sale (I got one for my wife at scheels for $200). Sitka WS is also pretty good but is camo, not quite as much down, and was discontinued a while back. For synthetic Kifaru LP is worth looking into
For truly cold conditions you’ll also want insulated pants and foam pads (like a cut up Z pad) for your butt and feet. It’ll also help to loosen your laces to improve blood flow.
Nah, light synthetic won’t cut it, even with the fleece and shell, not if we’re talking November or later in the Rockies level of cold