Please post your code and a link to the motor driver/data sheet.
There should be no difference between using Serial.event and if(Serial.available()).
If you can capture the driver return events with a second hardware serial port on a Mega, you should be able to do so on a Nano Every with Serial1. The default Rx buffer is the same 64 bytes on both.
It would be unusual to overflow a 64 byte buffer before you can read it, but if indeed that is happening, you can increase to 128 or 256.
Why do you think your communication issues are caused by buffer size and possible overflow?