Syd

It sounds more masculine than feminine though...

This is feminine. The masculine form should be Sid. It reminds me of Sydney, which is feminine. And then of course you have the variant Sidney, which is masculine. It makes sense to me.

I feel like the masculine form should be Sid, short for Sidney, as that's the correct masculine form of the feminine Sydney or Cydney.

Syd Barrett comes to mind, that's how I know this name, but it's not the worst association. He was kinda crazy but he was cool. I like this name better than Sid, because it looks better, but it's an okay name either way.