Floyd musical direction if Syd had stayed

[Atom Heart Sun]

That's highly doubtful in my opinion. Just off the top of my head I can think of how Syd was under pressure to come up with a new song and wrote Vegetable Man.. yes he could have decided to make something totally new at this point, but stuff like Elephant from the 2nd solo album are a minority.
Another example: look at the lyrics from before and after the Floyd split, they are different in spirit with far more darkness afterwards. There's no way he re-wrote all the lyrics to all "old but as-yet unused" songs after the split.
I'd say at least two thirds of the solo period must have been written afterwards.