First, as you have found out the border has been closed since March 2020.

Second, the info in the COVID thread here is updated faster and more accurately than the UK's FCO advise . As is the information posted on the Overland West Africa and Morocco and also the Overlanding North Africa facebook groups.

Third, most border information at the moment published by the UK FCO, embassies etc is focussed on flying in and out rather than land borders

Since the border closed, there have been 2 confirmed tourist crossings over this summer, one heading North and one heading South. The guys heading north had medical reasons, support from their embassy and a huge amount of patience, it took them 2 months to get across the border.

The people heading south got a Mauritanian national to drive their vehicle south across the border. The owners had to clear the car with customs at Guergarrat, then return to Dakhla, fly into Mauritania, then travel up to the border to process the vehicle with Mauritanian customs. Apparently it wasn't easy or cheap.

Details of both crossings can be found in the Overlanding West Africa and Morocco group on Facebook.

Shipping from Morocco to dakar is possible but you would need to go back north to Casablanca or Tanger, its unlikely this is possible from Dakhla

Nobody knows any news about when the Moroccan land borders will reopen, everyone is guessing at the moment. Next announcement from the authorities is expected in the next couple of weeks.

Personally, I would spend the time enjoying Morocco and hope for some news over the next month or so.