sheet metal and "kinda think steel" are different beasts.
How close tolerances are we talking about? Does it need to look awesome or is garage good enough?
assuming you really are dealing with thick steel, I'd try and talk a local waterjet shop into burning it for you. Shouldn't take more than a minute especially if you have the cad file already done. I have a local guy I have do these types of jobs for me. And I have a lot more tools that you do. It just comes down to a time and money thing. I could easily do the rectangle with just a jig saw and some lube and a file. But if it's thick metal it might take awhile. The waterjet cuts perfectly and it just looks WAY better than I can cut.
I know my limitations when it comes to cutting like a machine tool can and I have gotten much more used to farming that stuff out and concentrate on things that require my time and thinking.
But if you do want to do it yourself, I would do the rectangle with the angle grinder being careful to cut inside the line. A jig saw would make this a lot easier if you can spring for one of those. The circle might be easier to buy a hole saw and use that
We're not out here to rough it. We're here to smooth it . Things are rough enough in town.