How to improve success rate of finding business class award seats on United Airlines domestic flights?

I am planning RTW business class for 2 adults in late 2020 using my KrisFlyer miles. To do preliminary planning I have used KrisFlyer and United flights out to Jan 2020 to get a feel for flights and availability.
Have also confirmed I comply with stopover limits and mileage restriction. No trouble finding Bne-Sin-Rome, then Athens-Prague-Geneva, down to Rio de Janeiro and up to Miami. Trying to get from Miami (or Fort L.) to San Francisco is proving almost impossible. Back from SFO to Bne no problem. I have searched United and Copa airline sites with virtually no success for business using both direct and stops… Don’t really want to downgrade to economy for the 6 hour direct flight or similar. Or use my QFF points to get business award on American Airlines for this sector (as they are showing availability!).
Is this a common problem with United? I even joined their program but makes no difference to shown availability.
Otherwise would I get better reassurance by purchasing Award Nexus or similar?

I would recommend their other hubs such as Houston, Chicago or Newark for the greatest chance of finding their award flights, especially Newark for flying on their 787-10 with real Polaris lie flat business class to San Francisco. Give yourself some flexibility in your destinations and you can always purchase cheap flights to get between locations (ie Southwest Airlines).

Thank you for the hub suggestions. I had looked quickly at these but will explore in more depth. Because of the stopover restriction I am up to my maximum so i still need to use Star Alliance from Miami (or it may end up being Fort Lauderdale) to a hub and then on to SFO. A shame that United has limited availability - rather like Air NZ!