I'm thinking of selling my pixel 8 and before doing that I know I should wipe the phone entirely. I am cautious when it comes to handling my data like writing 0s to HDDs (even multiple times with some randomness) on PC whenever I'm handing them out. But I'm not sure about phones.

Modern phones (Pixel 8 included) encrypt their data by default: since factory reset wipes out the encryption key, there is no way to recover the data, so you should be safe.

Do sign in again with a random login so it's less likely to connect again with your regular account then factory reset again.

I know with some phones (lg specifically), a partition that recorded certain telemetry data (all your passwords, location, sms, calls, lol) survived full system resets.

https://xdaforums.com/t/mlt-apk-disable-or-remove-lg-is-watching-you.2187920/page-6

The only absolutely certain way to leave no data would be to flash the firmware and all partitions.

Because you have a pixel, you could flash graphene and then flash back to stock.

Remove any accounts first and reset device so you don't run into frp issues.

You might be able to overwrite the data partition with this custom recovery

github.com/leegarchat/twrp_device_google_pixels/releases

Well, full honesty, not no way. But it's hard enough that you really only have to fear state actors and even they will have a very difficult time of it.

To further expand on this, you need to pull the storage drive and use an electron microscope. So if your stuff is not important to afford electron microscope time then you're fine with wiping the encryption keys.

Thanks. I actually just did!

flash graphene back to stock