Hello,
I've made a little more progress, I've seen on forums that users do :
sudo btrfs subvolume list ./
The result is slightly different:

I can now boot on this image with the correct syntax:

Now I'm trying to delete the snapshots that are no good.
I tried the following command:
sudo btrfs subvolume delete /@test1
But it just deleted the entry, my test files created after @test1 are still present.