you say checked discs and pads, have you had the pads out or are you just looking?
Reason for asking is it's well worth checking the calipers slide freely and the pads are also completely free to move in the housing. If not they will perhaps struggle to operate, hitting a 'stop' causing your symptom where they deal ok with light town slowing but will resist a hard pedal effort and quick stop.
Also check the handbrake linkage extends when released, to the lever stop on the caliper, if it doesn't the caliper won't adjust and the parking brake effort reduces too, you can test if the effort has deteriorated, in a fashion, by rolling the car backwards and applying the parking brake, and both sides of the car should rise at the same rate, if only one does then the other has a problem, yes it works forward but dips instead, and to a lesser extent as it's fighting against the spring.
OEM pads for this are cheap enough if you ask for value line, otherwise factor stuff is ok but I avoid ebc anything as it's generally *****, certainly if performance is expected without cracking pads or lumps falling out of them, and general shiteness if that was a real word haha. A lot of bikers know they're ***** too. So, maybe distinctly average at best ;O)