Right after my last update I decided to more or less completely toss my original design, so the past few months I've been doing a lot more designing, building, and testing than I have updating. So there's plenty of progress to report.
When I finished the first version of the wrist/gripper it became obvious that the original arm wasn't going to be able to lift it, let alone a payload on top. The first segment, which has to lift the most weight, could just barely handle the arm, wrist, and gripper, and the motion was very rough. The second arm segment was doing better torque-wise, but the belt drive was slipping and it didn't look to be easy to fix. So that was 0-for-2.
On top of that, I was looking at a design in which every joint was made up of a unique set of parts. While (sort of) fine for a quantity of one, it made the design a lot harder to reproduce. Also, I belatedly realized the virtues of through-joint wiring and wanted wider channels through the joints for plumbing.
While the first design turned out to be a failure, the process had taught me many lessons to improve my workflow and the redesign went much faster as a result. First, I chucked the original frame design and used 2" tube instead for the arm segments. This cut some weight and a lot of part count. It's so much simpler I am kicking myself for not doing it the first time.
Second, one of the central realizations was that the robot was just a series of joints, and that I could (in theory) have most or all of the six joints be identical in everything except the motor (the joints have widely varying torque needs). At this point, I'm confident I can use the same mechanical guts for at least 4 of the six joints, and can probably reuse a lot of parts for the other 2.