I suspect with all the extra traffic from the FINAL FANTASY XIV expansion Endwalker, your one and only EC2 node in Australia is hitting congestion every night during Australian peak hours. This obviously makes RTT really inconsistent.
Australians tend to prefer this node over the others because there are not a lot of routes from Australia to Japan, and EC2 usually gives consistently good results. Any chance you’d consider spinning up a second node on EC2 in Australia please?