You're mostly right on your assessment - I don't intend it to be a good burner phone. There's the possibility to swap in GSM modems (thus changing IMEI) - except that the "manufacturer" part in IMEI is going to stay the same, or very similar, so it'd be like that story about a guy who sent in a bomb threat on his college campus through Tor, and it turned out he was the only one using Tor on the campus. IMO characteristics for a good burner phone are just like you're describing, and best practice is to throw those out or destroy them, after all.
And keep in mind that there's also the fact that you have to buy the phones somewhere - that can be tracked, whether it's a package from eBay with 10 similar phones or just a guy buying the cheapest prepaid phones in a supermarket.
And keep in mind that there's also the fact that you have to buy the phones somewhere - that can be tracked, whether it's a package from eBay with 10 similar phones or just a guy buying the cheapest prepaid phones in a supermarket.