Hardy geraniums are my favorites......a light trim keeps them tidy if you choose the right variety

The small spreading Campanulas are easy and keep very low. Particularly good for drier soils.....don't like wet cold winter soil.

My first thought was hardy geraniums too, but be careful which variety you choose as not all flower all summer. A variety of different ones would be attractive.

Don't know where you are in france but Erigeron worked for me there and Oenothera siskiyou always flowered it's socks of but can be invasive. Ceratostigma plumbaginoides is another good ground cover plant. Achillea 'Christine's Pink' or Yarrow 'Christine's Pink'

A good choice would be the hardy ganium Rozanne beautiful blue/white flowers that flower for such a long time, mine are still in flower now.

Yes they are great - one plant can cover quite a wide area. But here they don't come into flower until about midsummer, so you need an earlier flowerer to go with them.

Could you mix heuchera in too, the colours in the leaves can be like a tapestry and the spiky flowerheads a bonus

In southern France, Gazania krebsiana. Best with sandy soil.