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...
Maybe see some of his references first to see if they are capable of modding. And instead of going to the cheapest option, look for the most reliable and professional option. I hate it when others bring a bad name to modders.
the backlight is the FPC second from the top on the right-hand side daughter board.
when successful glitch, it should show green LED then off. Newer Lite sometime do not have the NO SD screen