K8y 1) their wifi router can be subpoenad to see all internet traffic, correct?
Yes, but I think it is more likely they would request that data from the ISP.
K8y 2) if one gets a new pixel and sets it up connecting to that same surveiled personal wifi, did you essentially just screw your privacy?
No. Connecting to a WiFi network reveals very little data about your device.
K8y 3) I assume they could see the new phone is connected to your wifi, and possibly its IMEI?
They would see a MAC address (randomized by default in GOS), and possibly a device name (like Pixel 9a) . I think that's about it. Certainly not IMEI.