> The marque feature either "50MB uploads" (Nitro Basic) or "500MB uploads" (Nitro). Is this per file, per day, per month???

It's per-file.

> I only share the occasional screenshot or small file, so uploads have never been an issue.

When I was running a 1440p monitor, screenshots usually ran into the upload file size limit. I would have to manually resize them. This was a hassle, so I started paying for Discord Nitro. I'm fine with it, I certainly get value from Discord.