How to indicate an End of a stream on USB
LoRa has a moderate data rate and typically substantial gaps between packets, so a simple thing you can do is send the data as printable ASCII hex with newlines or other terminators.

This will just short of double the amount of data you have to move, but do so in a very simple and deterministic way, and you can crank up the serial baud rate to handle it, especially as the baud rate only actually "exists" between two ATmega chips within the Uno.

Another plus is that the data becomes human readable, which is handy for debugging, especially if you might want to intermix advisory messages and data.