I think based on your threat model that buying in cash could protect you from big data knowing that you bought a pixel. Besides that I don't see the benefits. The gains are going to be from using grapheneos.
Paying with cash gives less knowledge about what you buy from bank/credit card/ data broker. It negligible for government because of the sim. Since you are willing to do a lot I would reconsider the esim personally because that seems really invasive considering location. Idk how expensive it is where you live but the silent Link and smspool data is much cheaper than carriers in USA.
I do think cash and even more so Monero are great clean ways to transact when possible.
Definitely don't let these considerations deter you from getting a pixel with grapheneos.