With 38x21, my lowest gear is just under 13 inches.

That corresponds to about 5.5 kph at 90 rpm.

If I am very tired I may drop pedalling rate to 60 rpm, which is 3.7 kph, about my lower limit for keeping balance.

That works on the loaded bike. With no front panniers, the front wheel lifts on very steep climbs.

Exactly, this happens on my bike also. That plus the "keeping balance at slow speeds" issue mentioned by Mickeg means that below a certain gear ratio, there's no point reducing further. We just have to figure out what that point is for each of us.

I think you can't switch to 36 spoke.  The shell is shaped for either 16 or 18 spokes on each side.

Could it have been a bad batch of spokes?  I think it was over a decade ago, Sapim had a bad batch of metal and made a lot of defective spokes.  I assume your spokes were much newer than that batch.  That bad batch, the spokes often broke in the middle, not the ends.

The only spokes I have ever had break were on an early 1960s tubular tire wheel.

I got a dent in a spoke on my Rohloff wheel, see photo, but the spoke did not break.  My spokes are Wheelsmith which is no longer in business.  When I got home, I replaced that dented spoke.

The wheel was built in March by SJS so I doubt it was affected by a bad batch from a while ago. I'm blaming my weight and luggage weight combined with how I was riding over anything on SJS' side.

The ride to Tbilisi has already reduced my weight a bit, and going to be taking off another 10-15kg of luggage weight too, so I think that plus 36h should fix the problem.

I carry spare spokes anyway and they are easy to change, but I'm lazy and don't like getting off the bike to faff.

For heavily loaded touring, I'm not sure you'll see much benefit swapping 26" to 27.5, there's only one way to know, unless you can borrow some.
I'm a bit surprised you broke spokes, I've broken more hubs than spokes! Actually, I've never broken a spoke on a Rohloff wheel, was it a SJS build?

I think I'm going to stick with 26 inch now as 40mm is too narrow and 650b seems like it might be more complication for little benefit. Even though 26 inch is slowly dying, I'm confident that it will still be more available than 650b.

Both wheels were built by SJS in March after I decided to switch to disc brakes rather than rim brakes. The rear was rebuilt because I wasn't happy going on the trip with the brake wear on the surface.

SJS quoted me for a Rohloff wheel build including changing the shell from 32h to 36h. I'm happy to eat the cost and slightly heavier wheel to minimise spoke breakages since I've got much rougher sections to come.

Not sure why I got broken spokes, as my previous SJS-built wheels had no problems. But myself plus carrying too a lot of stuff (which I'm going to be cutting down on quite a lot) likely contributed much more than anything on SJS' side

I once had a valve core unscrew when I was removing a Lezne pump hose. Since then each of my bikes has one of these fitted on one of the wheels to make it more convenient for checking if the cores are tight https://www.ebay.co.uk/itm/203317910489 .

I will be ordering some of those within a few days.  Thanks.  Brilliant idea.

I have a Shrader to Presta adapter on my rear wheel with a Shrader cap on it on most of my bikes.  Now I know what will go on the front wheel for valve cap.

If you decide to buy a large powerbank for touring, I do not know if this is a global restriction or only USA.  But in USA, powerbanks larger than 100 watt hour capacity are banned from passenger aircraft.

The 24 amp hour capacity mentioned above should be acceptable but that would be close to the upper limit.  That said, it is rare to see ones for sale over 100 watt hours.

For touring I have been using a 44 watt hour capacity one for the past half decade.  I keep it charged with my dynohub and Cycle2Charge charger.  But I am quite frugal with my power usage, phone off unless I am using it, etc.

I recently bought a Anker powerbank that is 20 amp hour rated, 72 watt hours.