A couple of points to note.

A. A hydraulic handbrake is not legal in a registered road vehicle as far as i know &...

B. The handbrake in the TRX rear disc brake system is a drum brake unit. The service brake is a disc & the whole thing is known as a 'drum in disc' system.

The standard handbrake, or parking brake as it is sometimes known, is more than adequate for preventing vehicle movement once the vehicle is stationary, as long as the whole system is in serviceable condition.
Have a good look at the cables & make sure that the inner cable has full & free movement. If they are stiff or siezed, then no amount of adjustment will make it work.