I have an A7R5 but I use Sony flashes. I think you will have to look to Godox to rectify this.

The A7R5 has dual minimum ISO. For shooting S-LOG 3 video, those minimum ISO's are 800 and 2500. For shooting stills they are 100 and 320. I don't believe the A6700 has dual ISO. Have a look at this video: Sony a7R V Dual ISO EXPLAINED! Best Low Light Performance ISO Settings

Problem with the Sony A7R5 and the Godox AD600pro, V860 ii and X3s. The high speed sync (HSS) only works in the camera's "fill flash" setting and there when you set TTL it fires 5 consecutive flashes instead of one. Furthermore, it works, let's say correctly (rather overexposed) up to a speed of 1/320s. From there, the more you lower the speed to 1/1250 or 1/2000, the more it underexposes them. The flash illumination remains stuck at 1/8, that is, as it was at a speed of 1/320, although there is the possibility of increasing it to 1/1 as is done in the manual setting. Can someone solve the underexposing problem in the high speed sync mode of the Sony A7R5?