[sfjailbird]

If I disable wifi on the shitty router my ISP gave me, and assuming the physical device is secure, am I safe from having my home network hacked into?

I assume the ISP could still backdoor their way in (is this likely?) but that is a separate concern.

[geocrasher]

This depends completely on what you put AFTER the router that your ISP gave you. You can often get your ISP to put the router into Gateway mode which turns off the routing part and the wifi. It becomes just a modem at that point, and you can use whatever good router you want.