Looking at the electric consumption of my 30 closest neighbors using an RTL-SDR, I'm still wondering what I should do with the information.

only ethical move would be to move your own consumption to when theirs is lowest and not retain any data but your own

Are the electric readings sent in plaintext or trivially decrypted?

Unencrypted, I'm sure.

I hadn't thought to look for smart meters, but my RTL-SDR (and my Flipper Zero) can pick up all sorts of temperature sensors, etc.

A quick google found this rtlamr project with a number of blog posts about people doing this with it.

https://github.com/bemasher/rtlamr