I don't think you will find Fusion360 easier to use than TinkerCAD, unfortunately. You certainly could model a phone case in Fusion though. If you would like to be able to make your own widgets and not rely on other people to interpret what you need, Fusion 360 and Onshape should both have good tutorial videos to get you started on creating solid models. The basic gist of solid modeling with Fusion 360, Onshape, Solidworks, NX, Creo, etc. is that you make 2d sketches and then turn them into 3d solids. If you provide the phone model and some rough dimensions, I can try and help you out with something if you're interested.