There is no known process other than impact cratering that could form the overall circular geometry and deep depression of Hellas.

However, plenty of things can modify that shape once formed (like the obvious zone of channels east of Hellas)

My take on the floor deposits of Hellas is that they are giant slump deposits (tongues, slabs, and folds) that have been eroded down into by the dry winds of Hellas and exposed in partial section as a series of scallpos and hollows with intriguing layering exposed.

This all relates back to Ken Tanaka et al (including myself) and our "mud ocean" model for the fill of Hellas and the Northern Plains, and the volcanic triggering of massive debris flows.
These concepts are written-up in the scientific literature.