Solved it. I can't just overwrite EVENT_OPEN_BANK to point to my own function, but I have to hide the original banking scene to call the functions.
Thanks for the help guys, the functions "WithdrawCurrencyFromBank" and "DepositCurrencyIntoBank" were the missing part of the problem
edit: Do you reckon the issue could be because I'm not emulating the interaction "INTERACTION_BANK"?
I haven't looked into "ZO_InteractScene", seems that I am unable to provide my own interaction for INTERACTION_BANK as it's already taken. Any idea how to unregister this?
EDIT2: I've got it! I replace SCENE_MANAGER.scenes['gamepad_banking'] with my own. Works flawlessly
Seems that the game needs you to interface with the game via the interaction "INTERACTION_BANK" to enable these functions.