..., has anyone worked out what the maximum extent is of each farm so as to be able to place each house X grid squares away with no chance for "poaching" each other's farmland, as well as no wasted land in between farms except for roads and Teamster offices? ...
Your screen shots are interesting. However, without the standard tile grid turned on, it's hard to tell what is happening. That's the green grid which is toggled on\off by hot key "G".
A long time ago, a player tested it out. The Farm planting zones are limited to five tiles in all directions from the Farm Building footprint. The results are different between field\"annual" crops and plantation\tree crops.
With field\"annual" crops, the plant, grow and harvest cycle is relatively short, so the farmers will plant a smaller area before they have to start the harvest. They select the best area (moisture & fertility), and naturally the number of farmers employed at the building will impact the size of the area planted each cycle. Generally a lot of the zone will be vacant, and you can't predict what part it will be. The planted area will be the fertile area without regard to distance from the building. It takes a good eye to judge the color so as to put the building on the least fertile tiles, but with plenty of fertile tiles around it.
With tree crops, one planting lasts many harvest cycles, so the farmers will - eventually - have trees planted in the entire five tile zone of the building. At least that has been my experience - there is no such thing as "sprawl." I don't know what is happening with the farm in your coffee screen shot. Are there four farmers actually available for work? I suppose there may be rare instances where the soil is so bad that nothing will be planted in some of the five tile zone.
The farmers have to clear non-crop trees, bushes, etc. from each tile before they can use it. In some cases, you can speed up planting by marking such plants for bulldozing so the Laborers get rid of them first.
A couple of hints.
You want your farmers to have housing as close as possible to the farm building (barn, eh?) where they are employed; so besides space for roads, you need space for houses.
Don't kill yourself with Teamsters ; remember the farmers will carry their own crops and are about as efficient overall as the teamsters if the destination is about twentyfive to thirty tiles from the farm building. When saving space for the Teamster's Office, remember that it should have their housing right next door. (You already know the Office needs to be close to the pickup point.)
Sugar cane is highly sensitive to rainfall patterns which form bands that change from year to year. Build rows of farms at right angles to those bands so you don't lose too much production at once. Really good sugar cane land is not good for corn at all - it drowns out. So you likely will not have intermingled cash crops and food crops -- that means you need Marketplace buildings spotted in the cash crop farming areas.
It's more important to reduce the walking connected with sleeping (to & from housing, rent is NOT to make money but to manage where people live) and eating (to & from food pickup points, mainly marketplaces since food farms, etc. do not keep stocks) than it is to overthink and overmanage the other needs which cause walking far less often.
BTW, so far as I know, no one has ever carefully observed (studied) closely placed farms to see if there is any significant "poaching" effect. In fact, in the case of field\"annual" crops - the buildings may need to be closer than the five tile zone would indicate in order to force planting of all tiles.
