That sounds like a really neat gadget, and might well be worth getting.
I normally have very, very few visitors except neighbours who wait until I'm in my front room and then rap the window loudly. It has the added benefit for them of a belly laugh as I jump, startled. In terms of friends, I'd say in a normal year I have maybe two visits from friends, although last year I managed three, I think, all in the garden. This year, I've had two.
But a gadget like that would be worth a go. When I lived in a flat, for a while I had a few grocery deliveries and had a doorbell with a light, but then I spent almost all my time in the bed-study, the kitchen or the bathroom. I didn't have many deliveries, though. In a house, it's a different kettle of fish.
As for the test and symptoms, it's not the lateral flow test you do for symptoms. If you have symptoms, you book the other sort of test (my mind's gone blank on what it's called) at a centre.
Lateral flow tests just help to make sure you're not carrying symptomless covid that you could pass on to others. That could particularly be an issue if you've been vaccinated, because if you did get infected, you'd be more likely to be symptomless or have such mild symptoms you wouldn't tell them from seasonal allergy or whatever. It doesn't help either that the main symptoms of the virus if you're vaccinated, particularly, I think, if you get the delta variant, are most likely to be like the common cold, with a runny nose and a bit of sneezing. (Obviously, some people get iller than that even if vaccinated, I'm just talking about the mild end of it.)