For most threat models, ordering direct from Google is your best option. Sure, your IMEI is tied to your ID but that will also be the case if you use any post paid cell phone plan, provide your cell number to basically any government or corporate entity, or just generally use the phone without top tier opsec.
If you need a true burner, cash from Best Buy is a pretty solid option. Remember to wait at least 90 days after purchase to turn the phone on. Don't do so at any location that can be tied to you. Don't have another phone with you at the time. Have a burner SIM not tied to you for the initial connection. Pull the SIM. Wipe & install GOS. Enable airplane mode. Park the phone in a Faraday bag for a few months. Complete setup. Keep the phone stored in a Faraday bag until you are ready to use it.
It should never be in the presence of your normal phone. Or at any location that can be tied to you. Be prepared to dump it after any use.
If entities with subpoena power, warrants, and nuclear weapons are part of your threat model this really isn't the best place to get advice on opsec. And no technical solution, including GOS, will be enough on its own to protect you from those types of threats.