FYI the 2.5V on the serial pins is expected, and usually doesn’t cause a problem (was fine for me when I used an ESP32, for example).
The M7E has an internal voltage regulator, and the IO high-level is the same as the voltage regulator output. The voltage regulator only outputs ~2.5V when the module first powers up, and then changes to 3.3V once you start using the antenna (eg. start reading tags), and which point the IO pin high-level increases to 3.3V.