All -
For the sake of others with the same problem, I was finally able to resolve the issue and write to the xEM chip. Here is what worked for me -
I am using a Proxmark3 RDV2 as shown above and was only able to read the chip with the included antenna. After searching through these forums in addition to the proxmark forums, it seemed like the only solution was a different LF antenna.
I found one that is essentially the same as the one sold in the xEM Access Controller kit.
https://dangerousthings.com/product/xem-access-controller/
I removed the antenna, spliced and soldered it into an MMCX antenna cable… And IT WORKED. After checking the new antenna with “hw tune” and “hw status”, I was able to both read and write to the xEM T5577 chip!