The counterpart to Solidworks is Autodesk Inventor; however, that's Windows only, there's no native Mac version. You'd need to dual boot (emulation *might* be acceptable, however it's highly dependent on graphics systems so it's best to have full access to your hardware). Like Solidworks it provides a parametric environment so holes match bolt size and location, parts can be intelligently linked, and so on. It's also better at handling detailed 3D solids than AutoCAD (note that it doesn't do detailed threads by default, as industrial standards make that level of detail irrelevent).
Backing up a bit, I do quite a bit of 3D in AutoCAD. My normal plan of attack is to look at what needs to be produced, how much detail is required, and how much duplication there is, before starting. Most especially I'm looking for "exploits" - places where I can duplicate, or mirror, or otherwise not have to do the same thing from scratch multiple times. For a mounting flange like the attached PNG, I would first work out the overall shape, including the major central void. I'd be tempted to include some of the exterior "bulges" supporting the bolting if they require fillets, although I might UNION additional circles later if that would work out easier. Then I'd start looking at requirement for bolts, including countersinking, and make one of each type. Those would get copied to the appropriate location before being used for SUBTRACT or possible SLICE to get the end result step-by-step.
All along the process, I'd be taking a copy-forward approach before each major modification i.e. take the work, make a copy next to it, then operate on the copy. Sometimes the operations don't go how I want, so being able to "step back" is handy. Many times I'm making multiple variants on a single option so that also helps when starting from a common base.
If you are going to fly by the seat of your pants, expect friction burns.
"I don't know" is the beginning of knowledge, not the end.
