Finally got my i5 back. As people here might know, I am one of the unluckiest since out of all of the i5's in the 1st batch, apparently only mine (AFAIK) could not update firmware no matter what method or update package.
@Andykong messaged me to give some possible methods of fixing it but none worked so as a last resort, I had to send it back to the factory.
Andy told me to send it to him and he'll personally bring it to the factory, which I did.
Good thing EMS was being fast this time and the return shipping was also very fast.
I just opened my i5 again last night and briefly tested it. Everything works fine so far and smoother than before.
It's not a new unit, apparently they replaced my faulty motherboard then flashed FW1.5 international version.
Considering they could do the firmware update, I assume problem solved, though I'll see it for real when next FW update rolls in.
Andy, once again I could not express how grateful I am. Andy and Cayin team has really great support which is a crucial thing to have for customers.
Thank you again for going through all those troubles yourself Andy, now I can enjoy my i5 again and just realized how much I missed during its absent.
Cheers
P.S: when can I expect 3m USB-C to coax cable?