Well the easiest and most secure way would be to enable https(TLS).
You can do that with a self-signed certificate or get free tls certificates from letsencrypt
Do you have good reasons for not allowing encryption and then failing to remind users that their data is unencrypted over the network? Even for AREDN networks, I'd recommend keeping the warning banner so no one makes any assumptions about the system's security. 2FA login and field level encryption was designed exactly for this use case. There's some encryption built in for things like passwords, where the intention is not to hide the purpose of the communications, but is designed to hide the actual values.
Over the past 10+ years, we've seen web browsers default to loading any URL typed as HTTPS before allowing the unencrypted version.
I can work on an option that reminds users who have administrative permissions that encryption is disabled. Anyone logging in with admin will need to dismiss the warning weekly. These changes are building now. it will take a few for the build process to complete.