Backtracking, I know, but: So, I don't want to go to a Eurovamp in France, DH doesn't want to go to Thailand. What's the compromise?
This is only a compromise for now - say, next summer, since he's already ready to book - but I just took a look at the eurocamps.uk site and there’s one, Parc Umag, in Croatia. It’s 9km outside of Umag, but it’s right on the coast in a very cool area - you’d be right on the border of Croatia, Slovenia, and Italy. There are a lot of interesting smaller cities nearby: Trieste, Rovinj, Pula, Piran. And the coast and countryside are gorgeous. Roman ruins, semi-castles, huge public parks/gardens, museums, old town centres, hills and mountains and rocks to climb.
If he's set on renting a car anyway, you could split the time between the campsite (which looks like it has an attached beach) and trips into the cities for the whole family. Or he could drop you off and pick you up at the bus station in Umag and let you do some day trips. (Not sure how old the other children are; maybe some would want to come along?)
Another option - a lot more research, but you could do it online if you have time - would be to fly into somewhere like Trieste, Rovinj, or Pula and rent a vehicle big enough for all of you and drive down the Croatian coast staying at campsites (not from Eurocamps, but whatever’s available) or whatever other accommodation fits your budget and stopping at beaches or cities or whatever else you want to see as you go. (You might even find hostels on the way that have 6-bed ensuite dorms and have a big room to yourselves, if the youngest doesn't need a separate bed.) Plan how far you’ll get so you can book ahead for accom and for flights home - they’ll be crazy expensive if you book last minute in the summer - but you could fly home from Split or Dubrovnik.
You could do similar a lot of places in Europe, I just used Croatia as an example because you mentioned it in your first post. I don’t recommend Norway on a budget - save it until you have more money! - but lots of places in Europe are gorgeous and cheap: generally the further south or further east you go the better.