The completely flat requirement could be a problem as most manufacturers mold ridges into the case walls to add strength. Samsonite makes a couple of lines that come close to your requirements: F'lite and S'Cure (most like the Oyster). I have a couple of Oysters and an F'Lite - the F'Lite is not flat inside and that is a pain.

Delsey has two lines that may tick all your boxes: the Delsey Honore and the Delsey Belfort Plus. There is also the Logel Octa line.

Do you mind me asking you why you have set out these three requirements? I am asking because these are the exact same things that my wife and I require - and it's nice to see that we are not alone.