You might consider having your wife doing the “training” in the Echo app. It can make a big difference. We have literally about 20 different people using the echo at our house, and have not noticed a gender difference. But there are some individual people who for whatever reasons may not be as clear.
As far as what you’re talking about, absolutely, you can use beam detection. That’s fairly common in some secure facilities, just like you see in the movies. The issue is finding some way of communicating it to SmartThings. Right now I don’t know of any device that does that, and it would take some fairly complicated set up.
On the other hand, you could also just use a battery powered motion detector. They’re quite small, will probably cost you less than the pair of beam detectors, and already work directly with SmartThings. You can use tape over the detection area to make the area of detection smaller if you want to, so that floor level pets don’t trigger it.
The following topic does include an example of a beam detector system that is used indoors, but it is not directly connected to SmartThings.