I am physically disabled and live with my aging grandma. What large dog breed should I choose?

I don't know if expanding my question breaks the rules, I've read them twice and they didn't assure me either way. So sorry for that.

Anyway, we live in a neighbourhood that is safe but has unsafe elements and a large (40+ kg) dog would be ideal for protecting the house. I have had dogs in the past, mostly being able to keep them in line and I really try to be a worthy pack leader but let's assume I'm just a beginner owner. I have muscular atrophy, I am weak and slow, I use a manual wheelchair and an electric scooter, my grandmother can walk but she isn't too sure on her feet, so a bumping-jumpy dog is not ideal. We live in a house with a small garden, smaller breeds I've had so far (dachshound mixes up to 20 kgs mostly) were okay with it but I've had an akita-dalmatian mix male and after him, a purebred akita-inu female and they both were a bit irritated by the lack of space, so I'd need a breed with little need for excersize, space, as I'm not able to walk the dog daily. I'll do my best to keep the dog fit especially during the puppy/adolescence period, but some days I just won't be able to.

My big lesson from having akitas was that even though I am capable of handling and overpowering a dog mentally, some breeds need to be overpowered physically too to earn their respect and I am not able to do that.

The one breed that keeps coming back during my research is the great dane but I'm concerned about the breed's general health and if the dog became diseased or something, that would be too pricy for us. What I do prefer about them is the mental sensitivity for people and yet being a confident guardian against strangers. The mastiff-like appearance also helps selling the whole "tough bodayguard" image.

Thanks in advance Reddit, for all your answers!

/r/AskReddit Thread