Layering system for PCT and 3 season hiking?

The really uber SUL gear lists you see are typically one season, but there is good information to be found in studying them. The season, local climate and the age and metabolism/cold tolerance of the user can make a difference.

Thru-hikers can change their kit as the season and local climate dictates using their drop boxes to fine tune the gear that is needed for the next leg, so you don't have to haul it all at once.

What is "lots of sleeping gear"? I think a 20F bag/quilt and a CCF or self-inflating pad are typical 3-season stuff. Using a bivy is more of a function of what type of shelter you pick. If you can afford a couple bags, you can trade a heavier bag for more water in the desert sections, etc. Lighter bags can (and should) be supplemented with clothing layers to get maximum bang for the weight. In a perfect UL world, you should be wearing everything for sleep at the coldest temps.

If you live in a northern state, this is a good time to do some cool/cold weather testing and find your cold tolerance. Go for walks in different weather, sleep in the yard, etc.

I would draw from a layering system like this (not all at once) for a thru-hike:

Bottoms:
Merino wool socks
Silkweight to R1 type long johns to suit expected conditions
Zip-off pants
Rain pants

Tops:
Silkweight to Cap3-ish base layer shirts
R1/Power Stretch mid-layer (hoodies are good)
Windshirt
100g type polyfill puffy jacket (or vest if you can hack it).
Rain jacket or poncho

Accessories:
Light fleece beanie or balaclava
Sun hat
Liner gloves
Glove shells
Gaiters

If you lean to being cold (and in dry conditions), get out the down.

If it's hot I wouldn't want to get caught in a long sleeve shirt, rolling up the sleeves would create very warm sweaty spots.

My current system is a 5oz tnf syn tee, a 2.5oz montbell tachyon anorak windshirt. A down 7.5oz backcountry.com anorak. A long sleeve thermal style shirt, for active use. And lastly a 2oz plastic poncho for waterproofness.

All of these things can be left at home or on really warm trips I just bring the tee shirt on my back and the 2.5oz windshirt. The windshirt is the warmest price of clothing for the weight.

"Is it foolish to think I could get by with a windshirt (houdini), down parka (mointbell UL down parka) driducks rain jacket and maybe some wind pants(montbelll dynamo) for the PCT? I rarely see such minimal clothing lists."

Subtract out the driducks for all but WA and ditch the wind pants and that is what I took. I normally wore a very lightweight long sleeve shirt similiar to Cap 1 and lightweight Sahara pants. My down jacket was also a Montbell UL so it was even lighter as well. If I were leaving today I would take the same layers but I would add my cuben rain suit because for a few ounces I pick up full rain gear and VBL in the event of serious cold weather at night. I could take this system with a 20 Gergen quilt down into the teens.

I have no intentions of doing the entire PCT, but I have done probably 900 miles of it or so. Take this with a grain of salt…

I generally hiked in a long sleeved shirt for sun protection. There are lots of choose from. I used a button up kind with a collar and it functioned well in the desert. I am not a fan of using sun block on long hikes (it gets on everything) so I liked having a breezy long sleeve shirt.

I had a Patagonia Capilene 3 zip top that was nice in the Sierra during the cold mornings. It was also nice to put it on when I stopped (my sun shirt was normally soaked in sweat).

My rain/wind shell was a frogg toggs jacket. I never used it unless I was hiding from bugs or resting atop a pass in the wind. I would probably add a sub 5oz wind shirt to supplement this if I did it again. I would not leave the rain top at home.

I had a Montbell Alpine Light down jacket that I used a lot when eating at night. I would take a lighter down jacket if I did it again. A Montbell UL Down + cap 3 would be fine for most rests. If it gets colder, just get in your sleeping bag.

My only pants were a pair of convertible hiking pants. I also had a silny rain skirt which I would not take again. My legs never notice the wind as much as my torso, so wind pants would not have been useful to me.

There you have it, that's all the layers I carried.

I had other hats, gloves, socks, etc, but I wouldn't consider that part of layering so I won't get into it.

As for sleeping gear, I only took a silk liner to help keep my down bag a little cleaner.

Which direction are you intending to go, when do you intend to finish, and when do you intend to start? I think this system would work fine for you as long as you hike relatively quickly so you don't get stuck in some early season snow/cold rain in WA in late September/early October. You also seem pretty confident and have experience with the system, what may be comfortable to you may be miserable to someone else. I personally don't like jumping in to my sleeping bag with wet clothes and would strongly consider sleeping clothes at least for WA. I have also found that what may be bearable for a few days can really wear on you over the course of a thru-hike so it is nice to have flexibility and options. I think a bump box of some extra warm clothes at Cascade Locks would be a great idea. Just send them back if your system is working well and you can beat the nasty weather.

"Is it foolish to think I could get by with a windshirt (houdini), down parka (mointbell UL down parka) driducks rain jacket and maybe some wind pants(montbelll dynamo) for the PCT?"

I don't know that I'd go with the wind pants, so long as you have long pants that you normally wear. I personally like to go with a loose long sleeved button-up shirt all the time, and long pants, hot or cold weather.

But that list of clothing items sounds good; don't know that you even need the parka version of the montbell down jacket if you don't already own it. Just do your best to sleep at relatively lower elevations in the Sierras, and don't dawdle so that you're finishing in northern WA in later October.

Of course you do need some other clothing items — gloves of some sort or even perhaps mittens. A warm beanie. Wool socks; two pairs I suggest through the Sierras.

The clothing you bring is also related to your choice of sleeping bag. I was glad to have a(n honest) 20F rated bag for the first thousand miles or so, and then swapped to a 32F bag after I got to Sonora Pass (Bridgeport).

In terms of evaluating how warm you sleep: note that *some* people have found that when they lose a lot of body fat along the way, their sense of what's "too cold" can shift a bit. Best I suggest not to push the limits too closely. I personally like to tune it so that I can survive whatever conditions can come at me, and be comfortable in most of those.