I´ve looked into this option the last years for the same reasons and found the options very sobering.

MSC seems to operate the container ship MSC MARTA from Dakar -> Lome -> Walvis Bay -> Port Elisabeth -> Cape Town on an infrequent basis (every 2-4 weeks, 25-35 days). I'm not willing and able to anticipate and integrate such shipping gap into a seamless travel experience.

Generally shippings from Dakar to the south seem to be first routed back north to Cadiz where a few of the more frequently running lines coming from the EU cluster (Hamburg, Rotterdam, Antwerp, UK) to stop for additional cargo before continuing towards the Cape.

With growing tensions in the Red Sea and recent Huthi rocket attacks on cargo ships, German news channel n-tv just made it news this morning that intl shipping lines like Hapag and MAERSK announced to avoid the area and will take the old route around Cape Town again. Though this will increase traffic and competition along the western route, I´d doubt the prices to adjust down to more travel friendly levels. I'd rather believe a new price hike in shippings and all traded goods from overseas plus further supply chain disruptions. (While in other news the Panama channel is running low on fresh water and has capped its capacity and raised pricing, making the southern detour economically attractive again)

I'd be happy to read more optimistic research on that topic from the people here.

Edit: Most seem to be concerned about getting visa for Nigeria, which seems to be available in Ghana with a certain application form, as a German couple recently (May 2023) found out during their full western route "Tangier to Cape Town" journey in a VW T3 Microbus. Though their vlog on YT is fully in German, maybe it's worth using translated subtitles to get detailed info about their obstacles:
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