I won't speak for the safety of the blades at 700RPM.
That 722RPM is when it starts making 5W. Not the RPM it will always operate at, or the max RPM.
How strong will the wind have to be to reach 722RPM with 6' dia? Strong.
My rough guess is it will not reach charging voltage until 10~14MPH (without the rear set of blades).
Adding a second set of blades behind the first set is going to make it worse.
Guessing a 6-blade set of those should be on something with a cut in voltage at about 225RPM.
Speaking in generalities, with Ameteks as the example (should be able to find a lot of related data).
Most people say try to find one with cut in voltage between 300-400RPM, say 350RPM. Because that is well suited to 4' dia blades.
People who buy one with a cut in voltage of 500-600RPM have a fit when it almost never reaches charging voltage, and they need to shorten the blades.
Now relate that to your proposed system.
The cut in RPM is over double a decent Ametek.
And the blades are longer (meaning less RPM).
It would be workable with a 12V system, but that is an awful lot of mass and size for 150W peak.
G-