Meshtastic is very much insecure (except for the encrypted direct messages they added recently) by lacking Perfect Forward Secrecy, has a limit of 38 devices on a network due to its use of flood routing and can only operate over LoRa.
I would very much recommend using
https://reticulum.network/ instead due to its Perfect Forward Secrecy, it's current implementation supports billions of devices (could go up to galactic scale if necessary with a one line change) and can operate over any communication system which provides "at least half-duplex operation, and provide an average throughput of 5 bits per second or greater, and supports a physical layer MTU of 500 bytes" (rabbit hole warning)