It's a 2.5D part and you've modelled it in 3D and are using the 3D model in CAM?
There's your extra work! :D

To generate the paths you need in CAM for true 3D parts you will at some point need additional geometry as well as your 3D part, this geometry can be as simple as a perimeter to limit a cutting path to exactly the area you want or as complex as extending a compound curved surface beyond the edge of your model to allow a smooth finishing tool path to transition off and back onto the part for an axis feed rather than stopping exactly at the edge and creating machining artifacts.

Similarly for 2.5D parts you rarely need much if any part modelling and can control your tool paths with very simple, easily constructed geometry, this is why there are still people out there who think CAD/CAM for a single simple part is slower than manual machining, most often it's not ;-)

Just draw all your geometry at Z zero and generate paths to your required depths for
1. Facing cut
2. Rectangular Pocket
3. Through Hole

The geometry for your facing cut will be larger than the part perimeter by just more than your cutter diameter
The geometry for your rectangular pocket will be larger in two directions by just more than 1/2 your cutter diameter
The geometry for your round pocket will be unchanged.

In my CAD/CAM that takes me around 8 minutes to draw and tool-path

Modelling simple 2.5D parts in 3D is not required unless -
You need the part for a complex 3D assembly in CAD,
You can't visualise the part without pretty pictures,
The customer wants pretty pictures,