Layering and wind cutting is key for me.

For layering while moving I use poly or smartwool of lighter or med weight, and I replaced my fleece tops, coats, layering with 3oz Montrail pullover 1/2 zip windshirt. traps loads of heat, and still breathes well. I repeat 3 OZ! Especially warm if I add a less breathable wind/rain shell over it.

For real warmth, especially once I stop moving I my Cocoon style non-down insulated pullover 1/2 zip top and bottoms & and when sleeping if needed. Probably among my best UL conversions from traditional outdoors/backpacking/camping. Works like a down coat/pants. Add a wind/rain shell for even more trapped air/rain resistance. Not cheap but I swear by SUL synthetic insulating top/bottoms. I bought synthetic insulating layers top/bottoms as a hedge. I love my down sleeping bag, but after reading lots and thinking about the suggestion: do you really want everything you carry to be down? I decided NO in case everything else is wet, this will still insulate far better than a similar down top/bottoms.

My Cocoon style pullover 1/2 Zip Patagonia Micropuff synthetic insulated top is a "heavy" version & weighs ~12oz, my bottoms with full side zip weigh 14oz. Both are wind and water resistant & weigh FAR less than any fleece or sweater I've owned, but are also MUCH warmer - like wearing a down coat or pants. They are not as durable so I'm cautious if doing any camp work, etc. Cocoon versions at Backpackinglight.com weigh ~6-8oz ea. for pullover top & bottom depending on sizing.