That 40-20F sliver?

Sounds like you talking insulation while not hiking. Long john top,fleece, puffy, rain jacket, go to bed. In that order for me. Only thing that differs is how heavy puffy, if at all.

What MB asked, are we talking hiking, or stationary? Stationary in 40 degrees vs. sitting are two different worlds.

Indeed but how would you go for either?

While active I dare say I'm wearing a light wool tee and shorts. While stopping I'd toss on a down sweater (Marmot Zeus). Stationary would be long pants, an R1 hoody over said tee, and the down sweater (ignoring hand and headwear, I don't think you're asking about those). 40-20 is the perfect temp… Not too hot, not too cold.

So if I were you, backpacking, I'd be wearing long pants and a woolen tee during the day. Possibly a Houdini if the weather's wacky. And in my pack would be a light midlayer and a down sweater of between 2-4oz fill.

For 20f moving I use a Cap 4 or a Mec T2 Hoody and a Paramo Valez
adventure light smock, for around camp its a Montbell Mirage
and a Dead Bird Nuclei.

for 20 F moving, I wear nylon base layer, WPB jacket, fleece hat, walk very briskly because I'll be cold initially.

puffy when I stop

I never go below 20 F.

I don't get the fascination with wool. It absorbs more water and dries slower than synthetic, and wet insulation is poor insulation no matter what so I'd rather have something that dries faster or absorbs less water. I like a dollar store synthetic blend long sleeve button down shirt. Throw on a light fleece and wind breaker or rain jacket and you're good down to the 20's or teens for moving or 40's for camp. I use a light down puffy as part of my sleep system for cooler weather.

I'm fine with just my rain shell above freezing. 20-30, I'll pack a puffy for breaks.

I stay in shorts unless it's particularly windy and/or wet in which case I may pull on wind pants. My top would be long sleeve base layer and perhaps a wind shirt. This is while I am on the move. At stops I may throw on my insulation which would be typically my down vest unless I have traded it out for my hooded puffy as I don't carry both unless it is deep winter.

Wool is less thermally conductive when wet, it feels dryer. It's good for situations when you have zero breathability like a wetted out wpb jacket or a non breathable rain jacket on all day long in which dry time doesn't matter.
Also, evaporation takes heat away and sometimes it's nice to have something that dries slower so you don't get chilled every time you stop for a break.

To answer the OP question, you want some kind of active mid layer that's easy on/off and a heavier puffy for stops/camp.
I like a regular fleece vest for easy on/off.
Having a beanie easily accessible is nice, a beanie will add a bunch of warmth on the move and is easy to take on and off.

You also won't smell like a butt after two days backpacking :). Wool allows me to grab a beer at the bar after a weekend camping.

So layering a s/s and l/s Echo OR baselayer with an R2 fleece, shell mitts and liner gloves, a beanie type hat and balaclava and a rain shell might do the trick down to freezing and a big thick down puffy coat below that then? I wish they made rain jackets big enough beyond a DriDucks XXL to put my Montbell Alpine Down jacket underneath but if I just use it below 30F I shouldn’t have to worry about the rain with it too much I think.

For around 20F an R2 (with the other gear you mention) would be too warm for me when active, possibly too cold when at rest..
30~20F for me is Mountain Equipment Eclipse Hoody (think Cap 4 hoody, but better) and a windshirt. If hard, cold wind then hard shell. But I like a bit more when at rest. So I would go with a thinner fleece as base/mid layer for when you are active, and then have Alpine Light, or a warmer puffy for at rest.

R2 for active use? Perhaps 5~15F or just walking, not working as hard – otherwise I would consider it too warm.

Everybody's different, but maybe you're overthinking.
I run warm, and wear the exact same clothes while active at either temp(Marmot Scree pants, OR Ferrosi hoody, Capilene 1 top, boxer briefs). Only difference is where the zipper is on the hoody.
The same down jacket I normally add when stationary is also comfortable at either temp.
The only thing that might change with the prospective temps in that range is the weight of the bottom baselayer that gets added for sitting around camp, and sleeping in.