My first brisket was so bad my wife asked if I was trying to smoke a shoe. So take this with a grain of salt. I ran 225 for years because that is what the internet told me, then got tired of babysitting a fire for 14 hours. Bumped it to 275 last summer and the flat came out better than anything I ever did low and slow. Wrap in butcher paper when the bark sets and pull when it probes like warm butter, temp matters way less than people act like it does. Worst case you learn something and eat brisket anyway.

Did you ever figure out if it was a purse or a scattered hoard? We had almost the same thing happen on a site in Gloucestershire a few years back, 30 odd coins in one 5 meter stretch and then absolutely nothing for weeks after. Everyone kept saying the magnetometry was off or the grid was crooked but I really think we just clipped the edge of a deposit that got spread out by plowing. The frustrating part is you start second guessing everything and digging slower like that will somehow bring the signal back. Then you find six sherds in a month and remember that most sites are just mostly empty. I still wake up thinking about what we missed a few meters either side of that trench.