It is not worth renting a car. In Eritrea there are very few asphalt concrete roads and fuel is very expensive, besides you cannot just come and get fuel, it can only be bought with coupons.
Negotiate a taxi rate in advance, there are no meters. For the sake of saving some money, let the taxi driver know that you don’t mind travelling with companions.
It’s forbidden to move outside the capital without special permission. And it is really checked. It costs nothing but they may refuse to deliver it. So, it’s better not to plan itineraries outside the capital.