My personal fear is losing my photos when performing a sync. iCloud tops out at 5GB, which you can't effectively manage.

It's far safer to plug your phone into a Windows machine and using the file system GUI to copy the photos to the folder you want. Using Preview to import the photos as if the iPhone is a scanner is another safe method.

The multiple user/multiple computer problem that you're referencing can probably be solved by logging in to the corresponding icloud account on the computer.

The free iCloud tops out at 5GB (which is low for storing device backups and photos; ideally, you should get the same storage as your device size for free). You can pay for more storage.

And if you don't want to trust iCloud with photo sync, there are other options (Dropbox does that, and Plex has a photo sync function if you want to host those files yourself).