I checked the solar charging capability with a 18 V nominal panel.  The MPP voltage for this panel is 15-16 V, so I added a 4.7 kΩ resistor to the MPP pads, which sets the MPP voltage to 14.6 V and put it outside.

I measured that the solar voltage was being regulated to 14.7 V, close enough considering part tolerances.  Note that the panel in the picture was for testing only, this panel is too small to keep a Pi running.  It kept the charge up jut barely with a Model B+ in direct sunlight, a Pi 3B+ was too much.  For practical use, you need a bigger panel, or use a lower power Model A+ or Pi Zero.

Next up, I tested heat generation under high load with high input voltage.  I applied 20 V on the input with 2 A load using my electronic load:

The switching transistor, which was a hot spot in the old asynchronous design with high input voltage, is running at 64 °C, which is just fine.  Other hot spots are the charging chip (not entirely expected), the pass transistor and boost converter (expected), and... what's that hot spot near the USB connector?  Oh, the charge LED current limiting resistor.

How much is that thing dissipating?  A quick calculation shows at 20 V input, this resistor dissipates about 0.36 W.  Oops, it's a 0.1 W resistor! :)  Gotta do something about that.  The 19 mA or so flowing through the LED may also be part of the reason the charge chip is getting hot, but most likely it's due to the built-in linear regulator that needs to drop 15 V at 20 V input.

I changed the LED current limiting resistor from 1 kΩ to 4.7 kΩ, which should keep the current low and the dissipation in check at 20 V input.  On the other hand, at 5 V the current may become so low (less than 1 mA) that the LED is very dim.  Let's check that:

Looks alright to me, the red CHRG and green PWR LEDs seem to be about the same brightness.

Time to redo the high voltage input load test.  This time though, I also discharged the battery all the way, so the system has both charging to do and the load to supply.  Not sure if it made much difference, but hey, let's try to do it right:

First thing of note is that the hot spot near the LED current limiting resistor is gone.  Good.  The switching MOSFET's temperature is about the same as before.  The hottest spot is actually pass resistor Q4:

Maybe I should investigate if there's a part with lower RdsON than the SSM3J338R,LF I'm currently using?  On the other hand, the temperature it's at isn't really much to worry about.

Let's see what happens if we bump the input voltage up to 24 V.  In the previous asynchronous design, things would get hot enough I feared for thermal runaway.

As expected, the switching transistor is the hottest spot now.  Too toasty for comfort, but it seems stable.

Let's be really mean and also bump our load current to 2.5 A at the same time!

Ouch, hot!  Seemed stable though.  But don't do this, that's just mean.  I just do it to prove the design has plenty of headroom.

One important thing to note in all these tests is that the prototype boards I ordered are standard boards with 1 oz copper, while production LiFePO4wered/Pi+ boards have always used 2 oz copper for improved thermal performance.  So I consider what I'm seeing here quite good, and it will only become better on a production board with 2 oz copper, lower conduction losses and better thermal conductivity.