Is my donor area over-harvested with 3500 FUE grafts?

Yes, the donor area may have been over-harvested. Another problem is the harvested area is higher than the permanent hair in the donor area. This suggests the possibility of a long term growth problem if your balding pattern advances further back. The number of FUE grafts that can be taken depends upon your donor density and your hair thickness.  The lower the density and the finer the hair, the lower the number of FUE grafts that can be harvested. Finer hair requires more grafts yet less FUE grafts can be harvested when the hair is fine.