the best brake fluid I have experienced is motul rb600 (660 has got an even better spec) it has excellent longetivity (low moisture absorbtion) and boiling point.

On my R, the OEM pads are carbon ceramic which I find good but they will loose their performance a lot in the situation you described.

You can improve the hot performance by upgrading to sintered but you will increase the wear on the disc.

try using the engine braking & the front brake to reduced the load on the rear brake