Have multiple copies
I’m saying this instead of ‘back everything up’, because it’s easy to assume that in the cloud/online = “backed up”. It doesn’t. If you can, get an external hard drive for your work. Encrypt it. That’s where you put all the fic you don’t want to lose, including stories from other people that you love.
Use separate identities
This might seem paranoid, but if you don’t want to risk the whole world connecting your fic to you, get a separate, anonymous email. Pick a fake nameIf you don’t want your boss/teacher/parents/friends/etc finding you, this is essential. I’ve found the Firefox Containers function is super useful for this.
Choose open source alternatives
Open source programs aren’t owned by corporations. The guts of the program is accessible to anyone with a computer, and in general they’re better about privacy. That’s why I favour Firefox over Chrome, and my new text editor is LibreOffice. (OpenOffice is discontinued and has major security flaws, so avoid that one)
Consider a VPN
This isn’t an option for everyone, but it’s worth thinking about. A VPN (virtual private network) is a great layer of extra security.
Above all, come to terms with the idea that online content is not secure. It seems better, but if Google decided to get rid of the content on your Drive, you’d have no way to stop them. Lean on the side of paranoia. At worst, you’ll have very secure fanfic for no reason.