Why won't it go straight,turns slightly

Is there any small rocks, debris etc in the tracks? Because the debris might slow down one of the tracks, therefore making the tracks unbalanced in power.

do both drive wheels have the rubber intact? To me it sounds like one is missing. You could also have a motor that is a little lazy or debris on the magnet clutch in the gearbox.

If the rubber “tracks” on the drive wheels are intact, I would make it run for a while with the drive wheels off the ground and make it go thru a few “forward + 99” commands, and see if the clears the clutch and gets the gearbox to move more freely Aside from that a complete teardown and cleaning of the gear box, which is very involved.