So, I will probably be home alone when I ctb and my relatives will only return couple of week after, and since i don't want my relatives to discover my rotting body I want to let the authorities know i'm dead, but obviously only after i surely died.
Is this possible?
Only 2 things come to mind:
1 - If the police have an email, schedule one to your email address informing them of what happened. The mailbox may be completely full, if they have several emails for different reasons (computer crimes, theft crimes, and even suicides) you can send it to a specific one that you think is not very full, there is a risk that they will read it after that they have buried you...
2 - The same but with a trustworthy friend, an email or a telegram message, this app allows you to schedule a message without paying (not like what's app) and it would be my choice. Is it a trauma for the recipient? yes, but at least you don't run the risk of the first option.
Take a look at your city's police website and tell us, hopefully they will have one for suicide alerts or something similar. A hospital or other government building may be useful because it is less crowded.