It is better to take your data quickly than it is to have it span multiple days. Doing it over this much time will have moving shadows and affect the accuracy of the reconstruction. You can turn off the slowdown in the Settings.
The vision sensor warning is something you need to handle in the DJI app. It won't affect the data that is collected but can cause the flight to get stopped prematurely.
The "Over Flight Maximum Radius" warning comes up with you have a value set in the DJI app. We recommend not having a value there or having it set to 10 km or something like that.
There is some curvature to your model, most likely for the reasons noted above.
The GCP process does not change the shape of the model. It aligns things to the lat and long points and then attempts to minimize the error vertically.
Taking out curvature is hard to do over such a large area. That is why we don't do it. You need a lot of super accurate GCPs spread fairly evenly throughout the area to do it properly. Most people are not prepared to provide GCP data of the quality needed to do this correction so we don't offer it.