No, but I meant if there's a way not to change variant.cpp
Delete Serial1 and recreate it ? (Only on sketch)
Or add other type of end transmission interrupt.
Unfortunately, there's no way of doing that without modifying the "variant.cpp" file.
As Arduino chose not to make the serial handler functions weakly declared linker symbols in the "variant.cpp" file, it's therefore not possible to override these functions in your sketch.
Like I mentioned, the other option is to create and maintain your own board entry and copy of the Arduino core code on the Arduino IDE. It's then possible to make changes to your copy of the "variant.cpp" file without affecting other boards.