To be sure I have it right you have worn out the Rear Brake pads after
2,120 Km ( 1317 miles ) { Replaced at 50,500 and worn out at 52,620 }, and
2,180 Km ( 1354 miles ) { Replaced at 52,620 and worn out at 54,800 }, and then
5,200 Km ( 3230 miles ) { replaced at 54,800 and worn out at almost 60,000 }
For comparison my Trophy has done nearly 11,000 miles ( 17, Km )
and is still on the original Rear Brake Pads, with a few hundred miles of wear still left on them,
so the wear rate on your Rear Brake pads is WAY too high !
When braking are you using the Rear Brake mainly or the Front Brakes ?
( The majority of braking
should be carried out with the Front Brakes ).
Are you accidentally dragging the rear brake by resting your foot on the brake pedal ?
Either of these could cause excessive Brake Pad wear on the rear pads.
As you have checked that the Caliper is moving freely on it's Sliding Pins,
then the wear can only be caused by the brake pads being applied to the Disc,
and this may be due to the Caliper Piston sticking.
Go for a ride, and stop after using the brakes several times
then put the Trophy onto the Centre stand with the Gearbox in Neutral,
and make sure the rear wheel can be turned easily by hand.
If it doesn't turn easily then the rear Brake pads are binding,
indicating that the Piston may be sticking.
IF this is the problem, it may be possible to overhaul the Caliper,
with a new Piston and Seal set ( T2020050 ), or the complete Caliper may
need to be replaced. ( T2023053 or T2025040 - the Parts Lisiting is unclear ).
Cheers

