The best way to deactivate the airbag is by using an airbag srs defeat resistor ( they use them on rally cars, for obvious reasons)
They cost around £15, but you'll need to get to the socket to disconnect the airbag and insert the resistor.
They are literally just a regular resistor, so if you know someone that has used a soldering iron and is technically minded, they could do it for less than £5.
This is basically the technique.
https://www.toyotaownersclub.com/forums/topic/140137-diy-airbag-bypass-for-iq/
You could of course just disconnect it, but without the resistor tricking the system, your airbag light on dash will stay on.
For ETC. You need to disconnect the plug at the back of the ETC card reader