This sounds more or less like a mechanical issue. Of course, you will never get a robot to drive straight 100% and as last resort you should do what @reflxshn said, but I think it’s better to get to the core of things to figure out how to fix the problem as much as possible, not patch it. Would it be possible to send some pictures?
Yes, this is correct. You can test it by running a motor with either pct or rpm value and applying friction with your hand – you will feel it maintaining speed. You can also monitor the voltage of that motor, and see the voltage increase when you add friction.
What this implies for a drive that is not running straight is that the problem is not a high-friction build error on one side. You can have a fair amount of friction on one side and still drive straight, as the high friction motor will just receive more power.
If you are not seeing a straight drive with V5, it is likely that the two sides of your drive base are not parallel.
We had the same problem but found that our omni wheels had been completely dug out by the axle. We switched the wheels out and made sure to make everything spaced correctly and tight and haven’t had much issues since. Try checking your wheels.