My iPad Pro 12.9 and ASK work very well on my lap - until my cat decides she want to be there (my cat always wins)you're going to want some sort of keyboard with a sturdy hinge (not a magnet like the smart keyboard). The SK only works on a table or on your lap if you sit perfectly straight and don't move much.
The only reason I keep the ASK around is because it is the thinnest solution on the market and my use case doesn't call for much typing.