Hi Dave and Geoff,
All chain is the best way to go. You might end up anchoring with coral or rock in the area. Expect to get some days when it blows +25 knots, when you need the weight of the chain to take the tension.
I have 100M of chain, and I regularly put out 40-50M as a minimum. But that is 48T of boat.
You’re likely to be anchoring in depths of 2M to 6M at low tide in most places. But allow for 10M of tide in the west Kimberley, and 3-6M in the east Kimberley. So 6+10=16 and allowing for 5:1 scope in a 25 knot blow, you could need 80M of rode, but unlikely.
Common sense would be to move anchorage if you know its going to blow. Go somewhere where its shallower and more sheltered.
I agree with your comment about the $2 joiner. But take the extra 35M of chain just in case.
Ross