I'm gonna be honnest, combining the specs you mentioned is almost impossible. First of all, off-road pants aren't designed to go out at -20°C.
You can always use extra inner lining, long shorts etc. but there's no pants I know of that is made for this purpose
Second thing. It's either waterproof, or not. I don't sell stuf which is a wee bit waterproof but really isn't, that's just no quality. Like you said, it's all qualitiy stuf I'm selling
There's only one pants I can think which will be good at some points, and excellent in other area's. And that's the Klim Traverse
First of all, I've been riding around Europe & Morocco with this pants, and I wouldn't wanna have any other, even if it came for free.
I've been on winter camping trips using the Traverse, and if combined with an extra layer of long underwear it's no problem. Mind you, -20°C isn't common in Belgium, so the coldest I've ever used it was probably -5°C, and it felt great. Not too cold at all.
Venting is done by 2 long zippers at the front, and 2 exhaust zippers at the back, waterproof zippers off course.
Gore-Tex double layered, without an extra liner so the outside almost doesn't absorp any water. Doesn't cool the body down like other liner equipped pants do.
Over the boot cut, with velcro's so always the perfect fit. And it loosens if you get caught with your leg behind your footpegs orso, doesn't rip like other pants may do.
Only thing is the price
If you need a normal cut, the price is 319,95€, the Tall version is 334,95€. but the good news is that I'm giving you an AR discount of 10% (on Klim riding gear), and since you're the first one of this forum you'll get another 5% extra discount.
Making the Klim Traverse pants either 271,95€ for the normal version, or 284,70€ for the Tall version. Mind you, since it's a real enduro based pants, this price is without protectors, though the pants are equipped with pockets to use internal protection.
Trust me, once used to the Traverse pants, your other pants won't get much use no more
If you've got other questions and/or remarks, I'm at your service