When sketching a line attached to a circle, using a tangent contraint will make the transition between the curve and straight smooth.
Rather than sketching the inside wall and cutting it away, you can just do a shell/thickness command. Alternately, you can offset a sketch from the outside geometry rather than drawing all the features manually again.
Consider how the part is made, you have two very sharp corners on the centerline where the nozzle cannot create the geometry, these areas will be rounded by default, it is better to add fillets so you control what the machine does.