Needed freshness for long term statistics?

Hi guys,
i’ve got several meters that cannot be read automatically (they’re outsite of the house without any wifi or zigbee signal available).
So what i did the last years was going to the meters once a month, reading it, writing the value to my telegram bot which then appends it to an influxdb and have wonderful graphs in Grafana.

Since the energy dashboard is available i wanted to migrate this solution to HA.
But unfortunately all my calculations are wrong. It seems like HA doesn’t like it when the sensors value aren’t updated often enough.

What i can see in the developers tools is:
image

And this leads to very high and wrong consumptions in the energy dashboard:
image

I’m currently using a telegram bot in NodeRed which writes the data to an mqtt broker, which is then received by HA.

So I’ve got the following questions:
Why is the value going down to zero, when the sensor state hasn’t changed? (Picture 1)
What is the minimum interval in which i have to refresh the values so the data does not get corrupted? I’d assume at least once per hour?
Is there a way to automatically refresh the values (still the old value, but with newer timestamp) using automations? (monthly accuracy is enough for me)