Those sound like good options. I think the "last seen" is really useful on bot the node and sensor. I would only want to send notification for specific sensors or nodes and ideally the timing could be configurable, e.g. sensor x: 15 minutes, sensor b: 60 minutes.
Probably the custom script that runs every couple of minutes and checks the last seen is the most flexible approach.
Yes, node should not be sleeping when you trigger firmware update.
To avoid spam on log showing status for every 100 messages once. However you can enable Resources logs level to Trace level to get all the logs. This settings available under Settings-->System.
@ThomasDr I checked mixed sensor and it works as expected.
As you have grouping similar type of sensor data(volt parameter) you can use Grouped sensors graph will be more meaningful in this place. Update here if you still face issue.
@danilosouto You mean dashboard widgets update time? For dashboard we can configure refresh time. Sensor action default refresh time 30 seconds. you can change this under Setting-->System--> global page refresh