Replacing the slave cylinder is a fairly easy and relatively low cost job, so worth considering. I've replaced the damper with a Mitsubishi part and fitted a blanking plug which helped for a short time, but eventually a new clutch was required. I made a youtube video 'Hyundai i30 clutch after 250K miles' so you can see how it wears out. If the clutch master or slave cylinders are leaking, if someone presses the clutch pedal down while you watch the slave cylinder, and the release lever stays in the same position after a few minutes, they shouldn't be leaking. If the lever slowly moves back there's a leak. Or just pull the rubber boot off the end of the slave cylinder to see if it's wet underneath. If someone has already replaced the clutch I hope they would have checked that already.