I'm not convinced that there is a right or wrong time. The seasons may come early or late, they may be drier or wetter or hotter or colder than the year before etc. Last year, the short rains were longer than normal AND on time so we got very wet. so what? its Mother Africa - full of grace, charm and challenge! most of the people are nice most of the time and, whenever you go, you'll meet them and surprise yourself at what you chose to do and what you have no choice but to do with or without help. I expect others have been where you're thinking of going at the time you're thinking of and they'll have had their own challenges and triumphs.
prepare well and have a great trip. by preparation, I mean set up the bike, get your jabs etc and leave most of your kit at home. travel light and what mud you encounter (plenty) will be easier. eg take 2-3 T-shirts and buy secondhand ones along the way.