this is a rp2040 variation
read here to get an idea: https://gbatemp.net/threads/picofly-aio-thread.628951/
then read through the guide: https://gbatemp.net/download/a-definitive-picofly-install-guide.37968/
This looks like the one that not flashable. It works but slightly slower.
I would not try to flash it or even plug in the debug port without risking bricking the chip
Doesn't matter what firmware. There should be led light. I would get a multi meter and check all the points. Did you use a good mckroscope to check your work ?
I have done the full bga reball for my client. Takes much longer time using paste waiting for it to cool etc. I suggest doing the adapter reflow trick instead.
If it is a local deal, there is no chance they are paying up front and also handing them the switch (mostly paid after). Dropping off the switch at a personal house address/business address also gives a bit more confidence than dealing at a mall cafeteria/parking lot. A good lesson to learn for...