Hello ingohuber.

According to caseking, this aio has a copper cooler. This should work. It wouldn't work if the water cooling system had an aluminium cooler. Find out more about this.

The quantity is enough for several times. You only need to apply a small drop (maybe the size of a pinhead). You have to spread it on the cpu with the supplied "cotton swabs" so that it just reflects. And you must be careful not to miss anything, as liquid metal conducts electricity (short-circuits!). If the drop is too big, you can simply suck it up again with the syringe.

You can also apply a little to the cooler itself.

Oh yes... Use both hands to operate the syringe. Otherwise it can suddenly go "plop" and the stuff is everywhere. A bit of feeling is needed.

I don't know what the situation is with your i9 in terms of heads. Whether they are now soldered.

To replace the thermal paste at some point, you will need a fine sandpaper to remove it from the cpu and the cooler to some extent.

PS: Is the quantity enough?