When is too early to mow your lawn?
I don't think you can ever get a consensous on this issue.
Personally, I think, at this time of year, starting at 7:00 am would be fine. (I don't start until 8 am but, my yard doesn't take as long as yours.) Since you say there are no HOA rules and probably no law enforcement issue, this is truly an etiquette issue. You might upset some late sleepers but, you have to get this done and you are not intentionally making unnecessary noise. With lots that big, try starting away from other houses, maybe in the middle of the lot.