Yes, reformat. Cloud saves in my experience work just fine.

In addition to the other guy who answered you: Technically you don't necessarily have to reformat you hard drive to install Linux. Many installers give you the option to resize an existing drive and install Linux alongside windows.

Also according your steam library: As far as I remember you can export and import single games and you may be able to move your steamapps directory to your Linux installation but this may or may not work depending on your setup and your games. If you don't have a limited Internet connection I would recommend just re downloading the games your planning to play.

Cloud saves work fine whether on Windows or Linux.

It depends on the game and service.

But the best thing you can do is just go buy a new 1 tb os drive. Not too expensive, even for nvme, and it guarantees you lose no data. Then you have more storage once you actually know you migrated all the old data.

Plug an external disk and create a new steam library there, then move all your games to the new library. Now you have a portable library you can take with you on the go. Install Linux then add that library to the Linux Steam client. Now you have all your games back (including cloud saves).

You can also partition or use a separate drive and have both. I took the jump ship and reformat route, but you definitely don't have to

It doesn't require a reformat, no. It depends on what drives you have. It works best to have one drive dedicated to Linux though. In that case you install Linux on a seperate drive and point the Steam library to the existing Windows Steam library. Then you can use all the same game installations on both Windows and Linux and boot back into Windows if you want to.

There are a handful of mostly-older games that had native Linux ports by third-party porting houses which broke save compatibility between the Windows and Linux versions of the game. However, these old Linux native ports are generally absolute garbage and you're better off running the Windows version via Proton, which does have compatibility with your Windows saves as it is running the same exact game version. It seems most games with native Linux versions released by the actual developer are fine, it's just when they offload the Linux version to a porting house that it can get messy. Those old third-party ported games were typically from the original SteamOS/Steam on Linux era (2012-2015 or so) before Proton became a thing though.