I second Roxie's concerns about how much weight will be put on the dog. They shouldn't carry any more than 30% of their own body weight, and that only if they are conditioned to do it. Overloading is unkind in general and leads to excessive wear and tear of the dog's body which in turn leads to early retirement, and disability for the dog in old age. I don't think dogs are really a substitute for crutches. They can be very helpful with wobbliness and occasional mis-steps, but not as actual support for walking.
However, as you already observed, this isn't all that a service dog could do for you. Not having to bend over to pick up keys or a quarter you drop on the floor because a much appreciated blessing very quickly. Help transferring into and out of a chair or bed can be a big help too. Opening and closing doors when you're in your scooter. Operating buttons when you can't reach them, such as elevator buttons, automatic door buttons, and light switches. Picking up items off the bottom shelf at grocery stores.
There are safety issues too. One time I skidded down an ice covered driveway, sideways, and when the wheels of my chair hit the lip of the drive it flipped, tossing me into the street and traffic. I couldn't reach my chair, right it, or get back into it by myself. This was before I got a service dog, but it sure would have been good to have someone help me rise to my feet, drag my chair back to me, or help me drag myself out of the street. Eventually a passerby helped, but that was after several just walked past ignoring me. Fortunately I'm not in a wheelchair anymore.