I’m assuming a Linux install requires a complete reformat of the drive, aka having to reinstall / redownload my games, right?

Not necessarily. You can back up the game files or transfer them from another PC over the local network. You can also install Windows and Linux in different partitions on the same drive but Windows will sometimes eat your Linux install.

Also the cloud saves that I have stored on Steam, would those be transferrable / usable if I install on Linux?

Completely, yes.