Reticulum is a mesh networking protocol that can use a wide variety of interfaces – LoRa using RNode, AX.25, WiFi, or tunnelled over IP. It puts a heavy emphasis on cryptography. It is written in Python so needs a general purpose computer to run.
It claims ~300 bytes to establish a link and 0.44bps to maintain it. It seems to be link oriented but has a “group” destination type using a pre-shared key and a “plain” destination type using no key.