On a MacOS machine, is there any risk or trouble I'll encounter if I put some of my documents into a path that's not under my home ~ directory
Yes, very much so. You break the convention that the files belonging to a user are located in his or her home directory.

If you „retire“ a user, only files and folders in their home directory are saved.

Catalina, for example, plainly forbids creating folders in the root directory (/).

To make a long story short: There are reasons for the home directory. Do not fight those. I for my part enjoy the peace of mind this gives me. And the fact that I do not have to search all over the OS. Let alone Time Machine Backups.