all good points on more complicated motion systems. That's the kind of thing you don't figure out until you're actually working with it
The nice thing with this design is that I can adjust the arm lengths a bit to dial in the arm lengths to be as close as I can get them with a tape measure.
I'll post a picture of my design tonight, but I'm really happy with how it's turned out so far. It's based on a 75t HTD 3m belt. Zero back lash as far as I can tell so far. I ended up using 5mm bearings inside just riding on bolts to start, but I'm really happy with the results so far. The stationary gear is just free-floating without being fixed to a piece of 80/20 yet. I may add a built in bearing using 5mm ball bearings to connect the output side to the input. This might not be necesary once it's connected via pillow blocks and aluminum extrusion.
I found this guy who printed a harmonic drive that used a printed bearing raceway that looks really solid. He's printing the flexspline with a .25mm nozzle and dialing in his settings for PETG. I'm curious to try it. He's using it for very slow speed and low torque applications as drives for astronomy.