Wow, that is a big question. First of all, what material is it and do expect to machine the wholw thing on your 3 axis machine? The aperture with square corners will prove very hard to machine because round tools do not make square corners unless end on, so it is either radiussed corners, a right angle head or a subsequent broaching operation. Similarly with the channel behind the large aperture.
The sequence on machining will be critical as the part will not be very strong (in machining terms) after that big aperture is machined.
As for software, thay are all much the same in terms of machining operations available. Think of CAM software like cars, they all go from A to B, but some get ther quicker, some are more comfortable, some are easier to drive and some are not much better than a plank with four wheels and a clockwork motor. No software will make cutting this part easier, just easier to program. What will make cutting this part less troublesome is a lot of thought into the order of operations, how many release & repositions and most importantly, how are you going to hold it.
For myself, I would start with a block of material considerably bigger, square off the back face and drill the end holes all the way thro. Release and reposition onto a fixture that secures the part with the holes just drilled and machine the oval aperture and its face together with the other end face to depth. Release & reposition on the same fixture but wwith the otherside uppermost and face the same faces to depth. Profile the nearest edge.
R & R to an new fixture, machine the big aperture, drill & mill the square aperture leaving some on for the broach. Last kob of all wouls be to profile the side of the big aperture. This is a very sketchy production planning layout.