Assume the underlaying concrete is stable and solid, can you pour self leveling concrete and leave it like that for walking and as a garage floor?

Of the few brands I have worked with, although there are many out there, none were suited as a "wear surface" Otherwise, when done right, it makes a really smooth floor relatively easy!!

If self leveling concrete will do what you want in the way of a smooth floor, It can be painted to give the wear surface it needs. I would consider using epoxy paint, a 2 part mix that will dry/cure really tough, but you need to make sure the surface you have with the self leveling material will take the epoxy paint properly. I just used epoxy on a smooth 15 year old concrete floor and it builds up pretty thick after 2 coats. Perhaps this would do what you need all on its own?

Do look up how to use this stuff. I made/bought mixing barrels that will mix 2 bags at a time and made "rakes" to move it around to the places where it needed to go. Word of caution, it says it can go as thin as 1/8" or even feather edge, and it will, but it will not flow out level, it will leave lap marks where material has been pulled to a spot, drips off of tools will not level out if it is placed too thin. Thicker it goes down, the better the self leveling qualities it has.