Sydney is a complete disaster; expensive, competitive and stressful with desperate people throwing money they don’t have at places that are tiny, mouldy, falling apart with zero security. There’s no cap on rent increases and you could find yourself moving out after six months as there’s no long term options for tenants. Best would be Malaysia, an absolute dream. Beautiful plentiful apartments with great facilities at competitive prices.
Definitely move back. The fact that your cousins family offered a place for you to say earlier means they probably have a bit of insight into your situation and want you to know there’s a safe place to land if you need it. At 18 you don’t need your parents permission or approval so if they try to block you or insist you can’t go, don’t let them hold you back.
I know people who’ve lived in Korea/japan and there’s a big difference in the life you can enjoy if you struggle with the language. Since it’s not your choice to live there in the first place I can imagine it would be even more challenging. Are you on any 3rd culture kids forums? If not maybe check them out, see if you can find other people your age living in Korea with similar experiences. It’s good to get impartial advice from people who’ve had the same experiences. Best of luck
You coming to visit from overseas and your mom is asking your brother if you’d given him money like he’s an Uber driver is completely taking the piss. They demand you come back with the caveat you pay for everything and don’t stay with them or ask for anything is just setting you up to fail.
It sounds like they are punishing you for leaving and if you move back you’d doubtless find yourself an unwanted burden and scapegoat for any problems. You can’t win with people like that. Go back to Japan and make a new family with your wife.