OK, looks like things worked as I wanted. I just added
dtoverlay=disable-bt to the
/boot/config.txt file and it killed the onboard transceiver as expected. Running
rfkill at the command line shows only the wireless LAN as active.
One thing to note - the Batocera install (EmulationStation) still acted like it could search for and pair with controllers, but every attempt to do so failed. Would have liked for EmulationStation to recognize that Bluetooth was disabled and not enabled those items in the menu, but whatever.
Then I connected (hot-plugged) an external Bluetooth dongle via USB. Batocera/EmulationStation still wouldn’t recognize Bluetooth and pair to a controller, but once I did a full system reboot everything worked as expected. I haven’t exhaustively tested to confirm the lag issues are gone, but it’s likely that they are given how quickly pairing went with the external dongle…