Hopefully new clutch plates help. Although the old ones had only done 16.5k, they were 23 years old so that could be a factor. Good call on the oil, I'm a big fan of Motul 5000 semi-synthetic which the bike has always seen in my ownership. Fingers crossed for the next ride.
You might find the tiniest change makes that big difference. By the way are you using fully synthetic when the bike might benefit from semi?
The 90’s bikes rarely benefit from fully synthetic and I definitely got clutch slip even on the XJ with old plates and fully synth.