For the record Mauser is correct about the springs.
Sounds like you really just want a dagger, but can't justify the price for the one you would like to have to the wife without using the "home defense" angle..
A firearm is the best choice, but comes with a ton of responsibility (in terms of training, safety, children in the house, and legal use of course). Perhaps you already have, but I’d also highly recommend you research exactly what your local laws are regarding home defense, use of deadly force, etc.
Also, as a side, in general depending where you live, I suspect the law enforcement and in the end your local prosecutor would bat less of an eye if you were using something more mainstream for self defense (especially if you ended up killing someone) then perhaps an "exotic" weapon like a sword, spear, etc. Also in many places it’s legal to carry a firearm (if your state allows carry permits), but many states ban carrying large fixed blade knives, batons, etc.