Ooh AH. What a marvellous treat.
Surprised to see you getting through chains quite so quickly, tbh. I'm at a shade over 8,000 miles on both 'bents now, and they're still on their originals. Cassettes too, now I think of it. Less success with idlers, of course.
Not to say both bikes won't all fail imminently now I've written this, but everything still seems to shift just as cleanly as I can recall it ever doing. Not such a surprise on the chunky dual drive 8 speed SMGTe, but a hell of a trick for the KMC superlight 10 speed on the Furai.
Like the idea of the Alfine. I get on very well with the hub on the back of the SMGTe, and its ability to shift down at a stand still has saved my bacon more than a few times when something unexpected leaps out.
Watch out for the exposed cable on the bar ends. As you'll likely be mounting them upside-down, what would normally be a no-compromise weight saver becomes a very attractive point for water ingress. A glob of vaseline where the cable runs into the sheath will save you a lot of pain later on.
The SON hub is excellent. I've not tried the one sided version, but I really can't fault the install on either of mine. Given both are at the same mileage as the bikes, one has been dragged across a beach and the other survived a river ford, they're both still in regular commuter service.
What lamp will you add?
Incidentally, why the dedication to 20" wheels all round? Simplicity of tube and tyre spares, or are ICE rigid about other back ends? If I had a choice of non suspended 26/700 or plushy 20, I'd go the 20 too. But I'd be thinking about whether I could fake it with balloon tires.