Is there a through train from Cenxi to Pingnan?

Yes, long-distance bus.

Wuzhou Jinhui Bus Station → Pingnan Passenger Transport Center 33 yuan takes about 2 hours and 38 minutes.

Take the Cenxi-Wuzhou line (take 1 station) from Wuzhou-Cenxi Bus Station and get off at Wuzhou Bus Station, then walk to Wuzhou Jin Hui Bus Station and change to a long-distance bus and get off at Guigang-Pingnan Bus Station.