Don’t assume a large dog will deter break ins - we were burgled while my rottie was in the house, he didn’t alert us at all.
Of the large breeds labs and GSDs are usually the best burglar deterrent - not because they’ll guard, but because they’re vocal.
Giants, IME Newfoundlands are barky.
The big thing though to point out is that people are recommending things like rotties, labs and boxers when you said you wanted not energetic... on the whole, they’re pretty lively, they’re not the sort of breed to get it what you want is a couch potato up for a stroll.
Also - easy to get to a high standard of obedience does usually correlate with energy levels, less energetic breeds are harder to motivate.
I have some experience with deerhounds and Great Danes and I wouldn’t recommend either as sounding like they suit what you’ve said you want tbh, so I’ll leave the individual breed discussion to people that have them. But, there are some practicality issues with giant breeds, space and cost mostly - I love deerhounds, for instance, but I don’t have one because I drive a normal size car and I’m not willing to drive something big enough for that size dog, and they’re smaller than many giant breeds.