if your data gets hacked is
1) It is the root. Access to the data (reason1) and access to the server (reason2 that controls an equipment using the data).
2) That are the aim to hack.
3) If data is not secret, and for the technological sensors (periodically sending one-kind non-secret data) it's always true - no reason1 to hack the connection.
4) The data integrity is protected by API_KEY with a secret password at both ends. And data is ignored if API_KEY is wrong. And protected by the non-hacked server. reason2 is also not granted.
5) The server is controlling the data losing and makes the alarm.
So, i can make conclusion that the non-secret repeated and non-hacked data packet does not require HTTPS strongly.
And it does not need the server certificate to prove the host.
Doesn't this logical ?