Sorry to hear the chain in the routed slot isn't working out. I had great hopes for it. I guess keeping the alignment right isn't easy.
I wonder if VAWTs could borrow from something that came out of overshot waterwheels in their heyday: segmented gears. On these they used a gear that was the diameter of the wheel, but it was made of interchangeable/replaceable segments. These were probably made from cast steel that was machined for accuracy of the teeth. Even if the gear was 50 feet or more in diameter, it wasn't much problem for the segments to be hauled (by horseback or whatever) into place and bolted in. The wheel they bolted onto needed to be rigid, but Fitz was building steel wheels anyway. It was a big step beyond wooden cogs.
These would be heavy, but they could be made lighter because we aren't dealing with hundreds of horsepower here. I would think someone with a CNC plasma or laser cutter could make suitable ones out of 1/4 inch cold rolled plate. The pinion gear that drives the motor could be fairly long, maybe a couple of inches, to allow for warping of the wheel causing it to not be all in the same plane. The motor should be mounted on a spring arm that keeps it pressed against the gear to allow for eccentricities or out-of-roundness. Modern materials might be lighter and cheaper than steel plate.
It's hard to believe that something that was in use 100 years ago can't be bought today, but any Googling I've done has come up with nothing currently in production. Cheap fossil fuels and steam power were the end of waterpower, but now we need to rethink that.
I hope the manure spreader chain works out. I thought that chain looked familiar.
Alan