If your goal is food safety, then the temperature the bacterial growth occurs at is your primary concern - [the unsafe zone of above -5°C for fridges and -15°C for freezers over a total of two hours] - and that is why I explained about the thermal inertia concept. When you open your fridge/freezer, the cold air escaping will show as a big spike in air temperature but the food temperature, the bit that really matters, will only creep up a bit. That bit is what you need to monitor, as that is what the bacteria work with to thrive. That needs to be measured accumulatively over two hours to hit the food safety unsafe threshold. For multiple interruptions of shorter periods, you still need to track the unsafe zone period and discard when it hits the two hour threshold as cooling does not kill the bacteria, just makes it dormant. Above the threshold and it continues to multiply, not start again - cumulative process.
This concept is commonly misunderstood in the food industry. The clock starts from the time the bacteria start multiplying. You may need to monitor more than one food batch if you don’t operate on a FIFO basis and empty out your fridge freezer totally after a food spoilage event. The cost of an extra DS18B20 embedded deep inside your freezer to monitor temperatures there can also be offset by discarding less food that you are certain is not spoiled.
Regulating your temperature and alarms based on air temperature will result in unnecessary power consumption. The times when defrosting occurs (the compressor is turned off but the fans still work) shows that concept is well established in the HVAC industry.
If food spoilage insurance claims are to be pursued, either through the electricity supplier or the insurance company, a printout with minute by minute readings is going to be very hard to argue over. I know, having often had this discussion many years ago as an insurance assessor for food spoilage claims. No proof, no claim. See you in court if you want to argue. Brutal. Again, with an overexcited food safety inspector keen to assert their wide powers, the cumulative time in the unsafe zone meaning huge coolstores did not have to be declared unsafe, individual pallets of food monitored independently.
An unreliable sensor with such weak RSSI values and random reporting sessions relying on intermittent BLE packets is probably not going to comply with regulatory food standard requirements. You need to demonstrate robust measurements. This has health as well as wealth implications - you do not want to poison your family and customers, or risk public liability claims or fines from regulators.
You are probably using inappropriate measurement methods. Dump the Inkbird and go with a waterproof DS18B20 probe in each fridge freezer, inserted in a container of water for thermal inertia, connected to a single ESP32 GPIO using the one-wire protocol if the devices are in close proximity, and separate ESP32s if longer cable runs are not feasible. Is the time the food hit the unsafe zone going to be critical, or just that the temperature was above the limit when the power came back on? Often if you keep the doors closed, the insulation can keep the temperature stable for many hours, sometimes days, and your food is quite safe and the insurance claim will be denied, often on the basis of the fine print that obligates you to minimise the linsurance losses by not opening the doors (‘the fine print giveth, and the fine print taketh away’ is how it was elegantly put)
If you need to continue to monitor during power outage, battery backup via UPS is going to involve the HomeAssistant server, the network, and the sensors staying up. In that case a RS485 bus and remote power supply may be a robust option for connection to the central server location via twisted pair cabling. A simple RS485 adapter is only a few dollars and you treat them as an UART at both ends to transmit serial data, rather than BLE or WiFi your ESP can also use, but may be just as unreliable as the Inkbird you are replacing.
If you’re just doing this at home for personal use and don’t mind a severe bout of diarrhoea occasionally, yes, stick with your Inkbird and grumble to the sky trying to work around the inherent limitations of marginal BLE, relays and proxies. If not, get serious and put robust measures in place. After a few successful food spoilage insurance claims, a generator will also start to look attractive.
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