Realistically any distribution will meet your list of requirements. All supported distros are brilliant for security and privacy (well, except for (Red Star OS)[https://en.wikipedia.org/wiki/Red_Star_OS], but I’m sure that wasn’t on your short list anyways) there’s packages for Steam on basically every distro, and flatpaks make it easy to install other launchers on most distros too. Most desktop environments have a Mac-inspired preset as well

I’d say start with either Linux Mint for more general computing (it’s based on Ubuntu but excludes some of Canonical’s less popular changes to Ubuntu and ships with a more sane desktop environment) or Bazzite is the new hotness giving a very steamdeck-like experience with dedicated gaming mode or desktop mode. Their site appears to currently be experiencing some SSL issues though