Unfortunately most of the options that come to mind are not available for MacOS. I know this isn't what you asked, but if I wanted to reinvigorate a 2012 Macbo

Unfortunately most of the options that come to mind are not available for MacOS. I know this isn't what you asked, but if I wanted to reinvigorate a 2012 Macbook Pro I would look install something like Lubuntu on it, which focuses on being lightweight, and would run Falkon or Midori.