Switch to Copa/ConnectMiles
10x RTs to IAD are enough for *G with Copa (40K to qualify)
PTY-Africa thru USA mileage will be determined by what fare bucket you buy
What TA upgrades are you referring to as *G with United (unless you mean something like 1K, which is also *G)?
If you make Presidential on Copa, then you would get 6 upgrade vouchers which can be used on UA TA flights
PTY/IAD route is one of the harder ones to get comp upgrades on and is one of the few that I missed over the years