Hey Andrew, so glad you like it, it did turn out pretty good.
I generally finish my copper builds, and aluminum ones too, pretty much just like you stated above, once in awhile I'll add a few extra finer grit papers, and then, like you, always finish up with some Mothers polish.
When I do use a finer paper, I usually use the finer grits from these multi grit packs (see pic below) that I picked up at a "Hobby Lobby" hobby store.
I usually do all my polishing while I've still got the parts in the lathe, once in awhile I'll polish parts with a buffing wheel, especially if I need to polish crosswise to the rotation of the lathe.
This particular copper heatsink was finished a little differently, I sanded as usual, but instead of the last step being Mothers, I cleaned it with alcohol, and then sprayed it with a couple of coats of "Krylon" crystal clear gloss.
I use this method when re-polishing the heatsink would be a pain.