However, my utility provider may change the meter reading date, so the day of the month is actually in a numeric helper (eg input_number.meter_reading_day_of_month). Is there some way to put this into the cron line instead of hard coding a day-of-the-month?
You could do this instead: do not define a reset cycle for your utility meter, and use the utility meter calibrate service in an automation triggered by an input_datetime / input number to set the meter value to 0.
I was wondering if that was the direction I had to go, but thought I would check if I could eliminate that step and do it all at the UM level. If I take out the cron line entirely does that stop the meter from resetting?