Replacing Chain Question

Sorry, one more question, if I buy a chain that doesn’t have quick link, can I make it quick link by adding a 9s quick link as the last link?

In that 2nd picture, it looks like the valleys between the teeth have worn longer so the roller of the chain is sliding forward to bear on the back of each tooth. However, the only real way to tell if it is past it is to change the chain (10 min job) then try riding – if the chain skips over the teeth when you push down hard, then you need to replace the cassette too, or it will quickly wear out the new chain, and you’ll have to replace both (again).

In all likelihood, if the cassette has done 6000km, it is going to be done, and you could just replace it at the same time as the chain. Unless you’re on a really tight budget, this is probably the best option.

If you made 6600 kms without changing the chain then it’s not worth changing it now.

At this point the cassette is surely done and so are the chainrings.

Ride it is as is for as long as it doesn’t start skipping and then replace everything.

For the future keep checking the chain stretch level every few hundred kms, replace when it’s between 0.75 and 1% stretched.

6,600km is nothing in terms of distance on a groupset. Is there a reason for you to change the chain is it actually properly worn? Measure it. Don’t use a chain checker, use a tape measure. Remove from bike. Measure 100 links/50 plates, under slight tension. If 50inches then zero wear. If 50.5 inches then 1% wear, etc. my gut says you don’t have any reason to replace it, but you’ll never know either way without measuring. As for cassettes needing replacement every 6,000km? Do me a favour, that is a joke, unless you are most powerful cyclist in your area.