yes, info on Bulgarian Royal Decorations site is correct. In 1930-40s both Grand Cross and First class were awarded.
It seems Flat Reverse version didn't really impact Grand Cross, as it was given for only a few months. I have not seen any Grand Cross crosses with Flat Reverse.
There's actually a debate on whether Flat Reverse existed above 3rd class. Flat was only feature of the cross, and the cross was the same for 1st, 2nd and 3rd classes. You pair that cross with a star and it becomes a higher class.