Montane sizing can be a bit weird – I find their stuff very tight and restrictive round the shoulders and under the armpits so try before you buy.
Not insulated as such, but very toasty, is Paramo Velez stuff. Theres doesn’t seem to be enough material to make it as warm as it is – three layers of fabric but no filling or fleece. Very light, soft non-rustly feel and really good articulation. Compared to down jackets its easier to vent when you’re active, better in the rain than down too.
Although they’re thin / light you’d rarely need to wear more than a base layer under them for walking, but the fit i such that you can wear layers under if you need to