TH8 RS Short Shaft
I found this thing called a swage terminal, for boats apparently. Heck, what do those boating people know, it's a short shaft for the TH8 RS.
Shaft height reduced from about 140mm to 105mm. Just unscrew the shaft to switch between knobs/shaft. Longer for older cars like in GT Legends, and shorter for newer cars.
Alternatively, just get an M6 threaded rod and you can cut it to the desired length. Get some M6 bolts or bolt/washers too to fit along the rod so that the rod can fit snugly for the plastic sleeves that normally come with after market knobs.
Otherwise, the M6 rod would be too loose. Simple short shaft for Thrustmaster TH8 RS (video)
Note: I would recommend the alternative method, ie. M6 rod, simple, easy and hopefully easily attainable for you.
Breaking News: A customer just notified me that this Steel Gear Lever Shaft makes a great short shift rod. Great find!