I came the other direction, from Bamako in July. Here's my route and the notes from my michelin map: Bamako, Kita, Toukoto, Ouallia, Bafoulabe, Diamou, Kayes, Dag Dag, Aourou, west to Baediam (Mauritania), Selibabi
I know this is the opposite direction as you are planning, but it might help anyway. I also need to get it all written down before I forget it. I didn't go through Manantali as you are planning, but the route I took along the train tracks was a highlight of west africa for me.
Bamako to Kita: very good asphalt
Kita to Toukoto: very good dirt
in Toukoto, the good dirt turns north; to continue towards Kayes, find the dirt track that goes west.
Toukoto to Bafoulabe: dirt track that roughly follows the train tracks, sometimes difficult to follow, wash outs, would be difficult in heavy rainy season. I got lost many times and locals showed the way. Sorry, no GPS points.
just before Bafoulabe (going west), you can stay on the north side of the tracks and reach a car ferry or cross to the south side, follow the tracks, then ride the tracks across the river on the train bridge that goes into Mahina.
Bafoulabe to Kayes: dirt road, some rough spots, some construction.
Kayes to Dag Dag: the bridge north out of Kayes was under construction, so everyone was crossing by sailboats big enough for your big BMW. Finding the road north to Dag Dag was a challenge, roads were muddy and would be very difficult after heavy rains. A few stream crossings.
Dag Dag to Aourou: dirt road improves (less muddy).
Aourou to Baediam: can get your passport stamped out of Mali here, but the customs office is at the border town with Mauritania (can't remember the name). Asking directions from every person I passed was the only way I could find Baediam (and the customs shack, which was in a small border town). Two river crossings (50-80 feet wide, 2-2.5 feet deep at deepest point). Lots of roadwork on the Mauritanian side. Police (they let me camp behind the police office) and customs in Selibabi.
Enjoy the planning and good luck with your trip-