hi , i was on these roads in December/Jan and again in may.(dry season)so can
only comment about that time of year,i went to gambia in an old merc' in may ,the roads are fine till you cross in to gambia, then its pot hole hell to ferry,about ten kilks. i went off road to the barrage,not through rosso!
(hastle city) good track when dry,and a lot nicer at boarder.
the bamako route i was on Dec/Jan,again the worst road was up river in gambia,(the chassis broke near soma) this was on a old shogun,the new road is being extended as we speak,the best way is to cross to north bank ,new road all the way to grorgetown then cross back,i came from bamako the back way,and in to gambia by bassa santa , the roads in senagal are fine, in mali the chinese are building new roads as fast as i can type with one finger, came through mali on good roads, except for one stretch between bamako and boarder at kidira, but can be avoided,good luck.