WEIGHT is the most important consideration. Generally, it's worth paying for a heavier frame, and larger diameter discs. A 5 1/2' width would be the bare minimum. 6 1/2' would be my choice for your size tractor.
If you look at my mock up, the thicker arrow still leaves a portion of the original thin line. It's kind of like a Duplex scope reticle. It's only a suggestion. Your design is brilliant.
I figured out why this appeared in Spanish. A week ago I was in Etsy, looking for a magic 8 ball in Spanish. Apparently, it changed my default to Spanish!
The other (non bearing) approach I considered was a thin shaft though the middle that had pointed ends that rested in conical openings at the ends. Not sure how that would work in a dirty environment. That's why I was thinking enclosed in liquid
After reading your excellent description, the only thing I wonder about is how easily it spins (skateboard bearings). I guess I thought is was suspended in a liquid in that clear cylinder (kind of like a level).
You are way ahead of me on your design acumen, so I'll just sit back and watch.
OOPs, sorry. I wasn't looking for your intellectual property. I thought you were sharing an off the shelf build. You are close to a very attractive product.
I remember during the Gulf War (Kuwait) they were so good at taking out targets, they had to look at secondary targets. First on the list were Mime Schools and Accordion Factories.