asmith Your situation sounds similar to something one of my in-laws is going through. I'm sorry to hear it.
A pixel does need to be bootloader unlockable. Given the potential surveillance by your husband, Best Buy would typically be a safe place to buy one in person without delivery to your home. If messages are being monitored somehow, I would be cautious about location tracking too. Personally, I'd check for bluetooth trackers like AirTags (https://www.androidpolice.com/find-airtags-bluetooth-trackers-with-android-phone/) and leave every electronic device I own (phone, watch, etc) at a friend or family member's home before going to buy the Pixel. Leave any kids and their devices there too, if you can. You might even borrow a car if yours has GPS in it or if you find an AirTag on it.
Installation of GrapheneOS is easy. Ignore youtube videos for this and stick with the official instructions: https://grapheneos.org/install/web. People here or on Matrix can help you if you run into problems, though that doesn't happen very often for those who stick with the official web installer instructions. The most common issue I see is people using the wrong cable, so use the one that comes with the phone.
And if, weirdly, you are my in law, then call me out here in California. Either way, stay safe!