First off your wear seems excessive. I got 1800 km on my Giant with .75 wear. Incidentally both bikes use the same KMC chain that I found to be very durable and your bike has an XT cassette, mine is a SLX cassette. After the first chain was worn to .75 I did replace my cassette with a SunRACE cassette. Reason is my first one was replaced immediately as it was bent on deliver and the second one broke a tooth on the second gear so I lost confidence in the SLX cassette and I preferred the more even spacing between the gears on the SunRACE. My second chain (a Shimano eBike specific chain) on the new cassette wore faster and only lasted about 900 kilometres with the same care and lubrication. I'm still on the SunRACE cassette but back on the KMC chain and so far wear seams to be similar to the first chain.
As far as your loose cassette is concerned. That's not good and will lead to damage and probably excessive wear and bad gear changes. We also found that gear changes become less accurate and precise if the chain becomes dry and if you use dry lube, which we prefer here as a result of the dry conditions, you have to apply it more frequently than oil or wet lube. You need to look at why the cassette is loose because it may be more than just working loose. It could be a failure on the freewheel hub or hub itself. Either way you're probably in for a new cassette and chain at that percentage wear. Best to maintain and replace the chain earlier with less wear as it works both ways. If the cassette is worn too much it starts to wear the chain faster as well. Emtb's eat chains faster than normal bikes. More power and more weight.
