In a previous study, we successfully obtained a large-diameter, low-dislocation-density GaN wafer using the flux-film-coated and the multi-point-seed technique (FFC-MPST). As a production cost-cutting strategy, we are aiming to reuse a part of grown GaN crystals and produce thicker films by the Na-flux regrowth. Recently, however, it was found that threading dislocations (TDs) were generated at the growth interface in homoepitaxial growth of GaN crystals by the Na-flux method. In this study, we found that rapid growth in the regrowth contributes to the formation of inclusions causing the generation of TDs at the regrowth interface. Hence, we succeeded in suppressing the generation of TDs by a low growth rate, realized by a low-pressure condition at an initial growth stage. These findings are valuable for productivity enhancement of high-quality GaN wafers and help the widespread of GaN-based devices.