So, the 6-inch suction cup mount from K&F Concept is a real highlight for anyone who regularly films with a camera or action cam in the car. The workmanship is impeccable, the suction cup sticks so firmly to smooth surfaces (e.g. glass, bonnet, side windows) that you almost have trouble releasing it again! Particularly noteworthy is the combination of a strong vacuum foot (up to 60 kg horizontally) and the flexibly adjustable Magic Arm with ball head. The adjustment is quick thanks to a single wing screw, but is then bomb-tight. 
I've tested the setup with my Sony RX100 VII so far and plan to use it with the Alpha 7 IV, it wasn't a problem for this mount. 

The included adapters for action cams and smartphones make the mount universally usable, e.g. for Insta360, DJI Action or GoPro. The 1/4” and 3/8” connections, including ARRI positioning holes, allow additional accessories to be securely mounted without anything twisting. My personal conclusion as a full-time video grapher/photographer:
For anyone looking for a stable, low-vibration solution for shooting from the car, this mount is a real asset!