massive progress. Congratulations !
with a low 500dbm you need indeed have the tag nearly on the M7E board.
To your question:
could the low DB setting cause the “high return loss, check antenna” message?
no. Actually it is the opposite.
CRC errors means that the received information was incorrect. Each message has CRC bytes at the end that is calculated on the bytes in that message. The library is checking that and that failed.
This can be because of baudrate, bad wiring, bad power, etc
High return loss, check antenna: As the M7E is sending a signal to read a tag, it checks for the returned modulation on that signal done by the tag. If the M7E detects that returned signal is of a too high level, it assumes the antenna could not “send it”. This can caused by a bad antenna or cable errors, or antenna not able to handle the power. Reducing the 500dBm could make that problem happen less, but as a result even less distance between tag and antenna. This detection could also be wrong and caused by power glitches.
Based on the above I assumed a baudrate and power supply or the board connection to the antenna.
You tried the baudrate… maybe that helped on the CRC. For debug you could try to only change back to 38400 and see that it still works. Only for test, else I would actually leave it for the moment.
I am not sure whether you had reduced the read power ?
But it could also be the board. There have been others that had this problem, M7E Hecto not working with Arduino, M7E High Return Loss
I understood that you tried the Universal reader Assistent (URA) before. Does that still work ? (make sure set the URA switch back…)
Else I still suspect power could be an issue. The M7E (like the previous M6E) are very sensitive to good stable power. Maybe you can measure the 5V on the M7E. If you have another PC or better power supply you can try that. and else make sure to have the M7E power wire on the breadboard as close as possible to the MCU board. The less connections in between… the better.
BTW if you are considering external antenna’s you will need for sure external power supplied to the M7E.