Now to get a rear rack mounting USB rechargeable light to go with it :)
Super photograph, Andrew.
For the versatile sort of rear lamp that you apparently need, I think it would be hard to beat the CatEye TL-LD1100, which for many years was the GOAT, and probably still is for what you need. It operates happily for weeks on end on a single set of batteries (200 hours or more on the flashing modes), doesn't complain when one uses rechargeable batteries, has several flashing modes and a steady mode that can be seen from a very long way off, has dedicated side LEDs to cover all situations.
Does it have a downside? Sure, assuming that cost is less important than doing the job right, two downsides. First, you need to be aware that it is vulnerable to losing the cap over the batteries, which is also one of the side-lenses and includes the connecting plate so that the loss ruins the lamp altogether. Even though a variety of fittings (mainly for fitting to road bikes) come with the lamp, and others are available in the aftermarket, I tied mine to various racks (I had three of these efficient lamps) with tiewraps and just added one across the cap as well; the lamp is so economical of batteries that it caused no nuisance at all. Here you can see how I did it*:
(http://coolmainpress.com/miscimage/pedelec/338_rear_60_metres_zoomed.jpg)
The second downside could be more serious: It appears CatEye stopped making it, so you'll have to find one NOS. Before the -1100 I had the -1000 version, and the two lamps appeared to be the same. It looks like the Koreans -- who probably made the lamp for CatEye -- still have some in stock:
https://www.ebay.com.au/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=cateye+TL-LD1100&_sacat=0 (https://www.ebay.com.au/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=cateye+TL-LD1100&_sacat=0)
(https://images-na.ssl-images-amazon.com/images/I/61fAhqREhcL._AC_SL1000_.jpg)
*The other lamp on the rear of my bike is a BUMM Line Plus, in some respects a superior lamp, but no good to you: it requires a dynamo or a humongous battery, and, worse still, it has no flashing mode to preserve the battery, so isn't much chop for an adventure bike. That's why the Cateye -1100 is on the bike with the BUMM, to serve these other needs.