Thank you everyone for some amazing suggestions for me to check out, and sorry for the delay in coming back with more detail on Namibia for those who asked.
It was really simple to arrange ourselves. We had 11 nights in Namibia then flew to Cape Town and had 4 days there. If I hadn't been going to CT to see family I'd have stayed 2 weeks in Namibia I think. Because we were constrained on time we booked all campsites in advance as we didn't really have long enough to alter plans.
Flew Lufthansa via Germany - so left London on Friday evening, change in Frankfurt, then arrived Windhoek the next morning.
We hired a 4x4 with two roof tents from www.safaricarrentalnamibia.com - we picked that company as they had an option for 5 people (2 adults 3 kids) with one smaller roof tent and one larger. None of the kids are massive (if I had 3 large teen boys I suspect it would have been too much of a squash) but they managed in the larger tent fine. They picked us up at the airport and drove us to their place and spent a couple of hours showing how everything works on the car/tent/cooking equipment etc. Drove us back to the airport at the end of the trip
We then drove straight up to Etosha, and were at a water hole looking at giraffes/elephants/rhinos/lions later that evening.
In an ideal world I'd have done Etosha last (as it was so exciting to see the occasional giraffe or zebra or kudu at the roadside on our way up there, but by the time they had seen dozens/hundreds the excitement wasn't quite so significant and so I'd rather have kept it for last - but it was a deliberate choice taken because the rainy season was coming and we wanted to ensure we got there before it started as the animals don't come to water holes so much if there is rain everywhere).
We stayed 2 nights at Okakuejo and one at Olifansrus in the park. Very different camps and experiences. We saw more animals in number at Okakuejo but more close up and at a smaller camp at Olifansrus. With more time I'd have done an extra night at Etosha.
From Olifansrus we had quite a long drive to Spitzkoppe where we stayed 2 nights. Did a hike with one of the local guides on the day between. Very hot, amazing scenery, amazing campsite where you can barely see the next nearest tent. The local village used to mainly farm but droughts in recent years made that unsustainable so it is good that they can now support the village via the community campsite and guided hikes etc. We stayed here Spitzkoppe Campsites
I think from there we drove through Swakopmund (might have stopped to do some desert activities with more time but it was cold and cloudy on the coast compared to inland) and through the desert to Sesriem just outside Namib Naukluft national park stayed here Sossus Oasis, Namibia - Service Station & Camping at the entrance to Sossusvlei & Sesriem Canyon (sossus-oasis.com) Next morning up super-early to visit the dunes - did a couple of hikes up a dune and through Dead Valley etc.
Then a long afternoon drive to Mount D'Urban Campsite (mountdurban.com.na) which is possibly my favourite campsite in the whole world for 2 nights of chilling, hiking, eating delicious food (the owners will deliver home baked bread, homemade jam, bbq meat from their farm etc) and chilling out.
From there to Quiver Tree Forest - we stayed at Garas Park rest camp for a night - again went running/hiking and explored.
Then to Jansen Kalahari Guest Farm for the final night before heading back to Windhoek. Did a game drive around the farm at sunset with the owner.
What I'd have done with more time: a bit more time - longer at Etosha, an extra day at Sossusvlei, maybe a stop in Swakopmund for some desert activities
An extra week or more: up to the Kaprivi strip for very different scenery etc
Even more time: I'd go to Botswana as well (have been before in my early 20s on an overland truck trip through southern Africa and was awesome).
Useful websites for itinerary ideas: Namibia road trip - the ultimate 2023 itinerary +Map (stingynomads.com) and various others
Useful stuff: Dirt roads = punctures. Know how to change a tire! No phone reception in the vast majority of the country....print off directions to places in advance as Google Maps is not going to help you. Also plan where you'll buy food - we cooked (there was a fire pit to cook on in all campsites and they generally sold wood too). There aren't many eating out options at campsites other than the big rest camps at Etosha, and you don't want to be driving around in the dark and then have to get back and pitch tents....don't forget with roof tents camp has to be packed to move.
Hope that is useful :)