I’d go with a good quality traditional key lock fitted properly in a good door and frame. Security isn’t a single point task, it depends on many things so think like a bad person and do what you can to make your property a harder target than your neighbours.
A good lock on a bad door doesn’t work, neither does a bad lock on a good door. Everything needs to work together. If you’re likely to lose your keys or many people need access a smart lock may be an idea, but good probably isn’t cheap. If it’s just you and you can keep your keys safe, then I think tried, tested and simple kit is good.
Ultimately though, as ever, it’s your risk assessment based on your needs.
Not sure I understand the problem fully, but you want a circuit to operate when you push the horn button, without affecting the horn operation.
Chances are the relay coil is drawing to much power.
Have you thought about adding a transistor to your circuit? It would draw very little current from the horn circuit but should allow you to drive something else. - such as your relay. It would of course require you to do some electronics.