I've fitted these round vents exactly as you said you would like to do them. They fit without gluing or wedging as they have a lip on the top edge that will catch in position as long as you use the correct hole saw.
If your soffit material is of hollow construction I found it easier to drill...
The problem of brushing your own Aluminium is protecting the surface afterwards. The Ali will start to oxidise and over time look patchy unless you clear lacquer it, or preferably anodise it. The advantage of anodising it is that you can do it almost any colour. This will cost more than buying...