Hi @Gnome and thanks for this super thorough post 
We have a tutorial to connect a mailbox:
For presence detection, you talk a lot about this sensor, but be careful: in some rooms I think a traditional motion sensor is more suitable.
The millimeter-wave sensor works well but sees through walls, so potentially in small rooms there can be false positives if someone passes behind a wall — after that it’s something to test at your place. I would recommend starting with one of each sensor (one mmWave / one motion), and run tests to see what you prefer day-to-day.
If you want something cheap just for announcements, the Google Nest Mini is good; I bought a new one on leboncoin for €20. It works with Gladys!
That’s what I do at home and it works great! Telegram notification with a camera image attached to the message
You should like this, new tutorial today in the training: