I've now got both wheels and wheel assemblies off and the axle does spin freely. The axle looks to be pretty clean and free from rust or pits that would cause it to bind in the wheel bushing. I've also removed and inspected the belt and it doesn't appear to be dried out or cracked or shiny. I do have another, but I also don't think that's the problem either, as it doesn't look terrible.
I've removed the spring that was supporting the transmission in the chassis, but I can't figure out how to remove the transmission gear shift cable so I can try to manually change gears separately from the shift lever connected to the handle. I've produced a video to show what it looks like. Would you have a few minutes to review? I'm also happy to provide any other info - different angles or pictures, etc.
drive.google.com