I'd say Besra as well. It doesn't look bulky enough for Crested Gos, wing shape looks better (slightly longer and more pointed than the rounded wings of Crested) and the tail shape looks better (slightly more square, not as rounded at the corners). Of course, the moult makes some of these features a bit tricky on this bird.
I think Besra should be resident in Guangxi, so time of year isn't a problem. Observations in Hong Kong suggest that they are often a bit easier to see in late summer (August-September) and may start dispersing away from breeding sites into other locations.