Join multiple tables without overhead on the DB Server using PDO
You need to rethink the schema...

Plan A:  Have temp, humid, etc, as columns in a single table.  If the date_time is the same from all the readings, then that does not need repeating.

Plan B:  Have those values in a JSON string in a single column in a single table.

Plan C:  Have a 3-column table:  (unitid, reading_name, value) and fetch the 4 values from 4 rows, but in a single SELECT.

In any case, Id is probably not needed.  The PRIMARY KEY can be (or start with) (unitid, date_time)