I think if you have Bitlocker set up with a boot password, and you are careful to shut down when you're not using the computer, then you're probably fine. But in my experience most people use Bitlocker just with the Windows login prompt.

As I understand it, the FPGA is able to initiate the TPM key extraction, yes. So it's not a sniffer that has to wait for the user to input the password.

https://pulsesecurity.co.nz/articles/TPM-sniffing