What can be safely moved from /users directory on Mac?

You can move any files inside the Documents, Desktop, Downloads, Movies, Music and Picture folders anywhere you want.

Moving them out from the user folder to a different folder on the same hard drive will not make any real difference however.

Any particular objective you chase by moving them out of the home folder?

If you are planning on moving them out to an external hard drive, then the only caveat there is that if you use the Photos app to manage your photos, the PhotoLibrary.photolibrary file needs to reside on an HFS+ or APFS formatted drive to work correctly.

Otherwise, any files and folders can be saved to an exFAT formatted drive just fine.

Other than that, your files can be anywhere you want. There's no reason to think moving files in folders like documents and desktop would in any way hinder your ability to use the computer.

I sense there may be something else there that worries you about this move that you have not shared, but like with linux or any other operating system, your personal files can reside anywhere that works for you. The home folder is just a convenient place to keep them. Not sure why there would need to be a guide as to what can be stored elsewhere? Outside of the Library and Applications folders all your files can live wherever you want. For Applications even most apps can me moved to another folder, but note that the system will not find them directly anywhere else so may not appear in the Launchpad icon on the dock.

"Usual" documents like .txt, .docx, .jpg, .mp4 etc can moved out (but do remember to backup them!).

Photos, Music and Final Cut libraries can be moved but they must be stored on Mac OS Extended or APFS formatted volumes because non-Mac formats like FAT and exFAT might corrupt them! Just Option-start the app and navigate to the moved library's settings file or package.

I'd not move ~/Library/Mail unless it is very large and I know what I am doing then... The same applies to the whole home folder.

Click on About this Mac -> Storage -> Manage and enable Store in iCloud.

This will store documents etc on iCloud.

You can manage which files are stored on iCloud from System Preferences -> Apple ID -> iCloud -> iCloud Drive