Time to do the inline thermostat mod and eliminate that plastic crap.

The incline mod will eliminate that weak plastic piece and has a number of other advantages as well, but changes your hose routing. I have the inine mod from Extinct. (installing this weekend)
If you want to a stock nose layout the aluminum T from Carrs4x4 is nice, but you still have a plastic thermostat and plastic 3 way. You can get an aluminum 3 way from carrs4x4 but now the price is more than the incline mod and you still haven't gotten a new thermostat and you have a plastic thermostat housing still.

Word, it's on the way.

Why do you hate my family? TEMPT ME NOT GARAGE DEMON

While I think the inline is a superior option if nothing else the elimination of the plastic bits, it's still more than attainable to have a cool running disco with the stock set up and a 180 degrees t-stat. Both options will benefit from new hoses and don't ignore the heater core hoses as they are directly above the pass side exhaust manifolds (if no SAI) and can deteriorate and fail. My green disco had newer main hoses but the shop did not replace the heater core and one of them blew up on my wife while on her way to work...I had a set handy as I ignored them on my white one but after seeing how the cordage was completely non-existent I quickly swapped in new ones when I did the hg job on the white one.

Update

I've confirmed there is coolant coming out the overflow tank reservoir bleed hose after I've been driving at speed for a while.
There were new coolant/UV ink stains under the tube output hose that drops to the right of the coolant tank and down to the frame.
The engine temperature per my TorquePro ODB reader app has never read above 192 degrees at highway speeds for an hour in 80degree weather, or while off-roading slow uphill on a sunny day.

I changed the oil and there's not a drop of coolant in it. Clean as a whistle.

Does anyone know what could cause coolant to come out the overflow tube like this?

Tim's inline mod, Abran's aluminum Y pipe, green coolant, 180F thermostat = D2 should be good to go!

I have added a 180F thermostat. I thought it was odd that I'm still cracking 190, but I am. Was getting over 220 before, though.

Before I tear-ass through my engine bay, is the claim that my engine is simply overheating enough to make the coolant boil, despite displayed temperatures on the ODB?

Are there any other technical possibilities, or things I can troubleshoot?

Incidentally, I think that my front radiator fan against the grille isn't turning on, because I have never seen it on. Even when rolling to check at a gas station off the highway.
If the detected temperatures aren't high (<190) I don't see how this matters, but I'm all ears if it might

180F isn't going make your engine run at 180F, it just makes the thermostat open sooner so the engine doesn't build up heat as fast. With a 190F that means the engine gets up to 190F before it opens which is 10F warmer vs the 180F, and under normal driving that 10F makes a difference. Eventually under load you could get back up into the 210-215F range and it also depends on the ambient temp outside.

I have the inline mod on my 02 Kalahari, and it has a M8000 Warn Winch, TF Front Bumper, and TF Front Skid plate. I live in SETX where it's very humid, and even with the inline mod I can still see temps up into the 206-209F range. That is of coarse with the HVAC on full blast, cruising at 80MPH with 2 adults, and gear in it. Same trip for example at night at the same speeds, less traffic, HVAC not on full blast and I could see 188-193F tops. Heck on my way up to Austin, TX last weekend at 75MPH it barely crept past 182F. I've noticed the sweet spot for any D2 with the OEM 180F grey or Tim's inline setup seems to be 65MPH. That seems to be the threshold for the D2 cooling system IMHO. At that speed across all 3 of my D2's they'll run 184-193F all day long because the cooling system at that speed/load can easily deal with the temps being produced, go beyond that point, and your temps will eventually creep up. I've taken Tim's setup on 4 5hr trips and you can watch it respond quicker to load input. Temps will go up, then they'll peak, and then creep back down until the next time you pass someone, hit a grade, or stop n go traffic. One thing the inline setup without a shadow of a doubt does better = idle temps. With the OEM 180F I couldn't idle for more than 5min without the temps just going up, up, and up..... With the inline setup I've been able to sit with the HVAC on full blast for 30min without it going past 195F, something I could NEVER do in the past. Which means out on the trail it will be a lot better off when you are waiting for folks up ahead to clear an obstacle.

Appreciate the thoughts and I've considered the mod.
I feel like I'm watching my temps and I have not seen them go high enough to explain the coolant loss, unless the argument is I'm wrong and I actually am missing large spikes when I'm not paying attention, or I'm not missing them but they're not being recorded due to faulty sensors giving misreadings. This feels unlikely.

Do you have any insight into how to troubleshoot the overflow, before fabricate the suggested inline thermostat changes?
At the moment, I haven't confirmed that would solve the problem.