managed to get the iphone to work again. not sure what is the problem, here's what i did:
1. hold power + vol down button
2. connect to pc & itunes
3. update ios to 12.4

after update the boot loop still happen.

so i let the phone charged while it still boot looping.

after few hours, unplug n turn off using power + vol down button

after 1 hour  turn on and it work like normal.
but when i tried to backup, not enough space leh... no free 128GB space on my harddisk.

yeah.

but i'm not sure it helps or not, because the issue happens intermittently.