About a quarter open on full throttle is about right believe it or not... The one thing that the Busso has is a larger throttle body than is actually needed... It's bigger than most Japanese turbo performance cars with nearly twice as much BHP
There are a number of causes for erratic idle issues that don't show up as fault codes:
1. Thermostat, is your car getting up to temperature within 5-6 miles of normal driving and sitting at around 87-92 degrees?
If not then this can be the issue. The car believes that the temperature is nominal yet the engine temp isn't correlating with what the ECU fuel rate is seeing, thus running lean then rich as a balance process.
Change the thermostat which is relatively cheap and easy to d.i.y
2. Throttle pedal multi-plug connection behind the glove box is intermittent. Never a good, sound connection at the best of times even when new. Many an owner have replaced the throttle pedal on fly by wire systems only to find the erratic idle and throttle response still exists only to eventually find the multi-plug is at fault all along.... A temporary fix is to cable tie the multi-plug between the wires joining the male and female sides of the plug preventing movement. A more reliable and permanent fix is to cut the plug out altogether and solder/ heat shrink the wires together to make a far better and permanent connection... If a pedal is needed in the unforeseen future it can be cut and soldering repeated.
3. TPS tracks are dirty in the throttle body. The black box that is attached to the throttle body has lots of little plastic cogs that can ingress dirt via the outer cover... A clean up with carb spray and lubrication with white grease can suffice. It may be worth while though removing the TPS from the throttle body then you will see what look like semi circle peg that often get covered in dirt and oil deposits, this prevents a clean contact between the TPS and the butterfly position marker.. Clean with electrical contact spray which is less harsh then silicon lubricant spray and refit back together.
4. Unbolt the breather pipe assembly from the cylinder head and give the pipe and plastic union/valve a good spray out with carb cleaner, re-fit.
5. Are you sure that your rubber induction tube to the throttle body is in good condition with no splits and leaking in unmetered air?
Take it off, seal the orifices with inspection gloves and elastic bands and test it for leaks in a bucket of water like you would with a bicycle inner tube... If air bubbles escape, air can get in making your car idle roughly and run lean.
Don't rely on just fault codes as the "be all and end all" of all your car's issues. Use them as an extra diagnostic tool, but more often than not, a fault code will not show up... Use common logical sense and follow the fault from start point to end point along the the most obvious route