I dont have exact prior experience with your specific scenario. But i have been working with mxpc for many years and many clients.
My suggestion is to provide a dummy-non-existing ingress class so that the operator creates and manages an ingress that is not used (dummy).
Then create and manage your own ingress/service to make your pods accessible.
This kinda is a workaround. There is no better or more clean way to do this right now (that I am aware of).
Information about NEG : https://cloud.google.com/load-balancing/docs/negs