I’m wondering if it might be a matter of power, not length.
If you have a powered USB 3 hub, plug it into a wall outlet (duh) and place the hub in the middle, between the extension and the Pimax cable. Yes, some of the extensions you tried are active/amplifying, but they are drawing power from the USB connection itself, which would further reduce the power available to the headset.
A update for those having troubles extending the Pimax5k
After trying a dozen USB cables passive and active that didn’t work I decided to follow a suggestion to try a powered usb hub. Obviously it is a power issue with the cables. The same cables that didn’t work with their own power adapter worked with the USB Hub. Most powered hubs come with 5V 2A or higher adapter were as the active usb cables only have 5V 1000ma to 2000ma. The adapter that comes with the pimax has an even higher output. I have tried two different 3.0 usb hubs with most of the cables and they have worked well. Longest extension cable I have tried is 5 meters or 16.4ft
looks like they did not specify a max but it seems for DP 1.3/1.4 (HBR3) 5m is the normal limit and extending beyond this is not so safe
building your own cable is a problem because of the custom connector on the HMD side
also its not that simple as usb does need a repeater/hub when exceeding 5m
as you can read here even the normal 5m cable from pimax has problems and together with the DP and USB 5m limits its not just soldering a connector to a 10m cable, usb definitely needs a repeater/hub and when the DP signal for 10m also needs a active component it will get much more expansive then expected, maybe we will never see a 10m cable for a low price from pimax? (edit: i’m quiet pessimistic here, maybe to much, if a 5m extension including the plugs work, its at least possible to get a 10m cable as a whole - without the plugs - working, also i’m hoping for wireless, its harsh going back to cable from the vive/tpcast)
i extended my 5k+ connection with the following stuff (dc extension is not critical), the most critical seems to be the DP and i’d keep it as short as possible (7.5m did not work stable with the 5k+, the 5m was tempting but as i only need ~2m ordered a 3m DP extension)