Comment by mh-
Not an answer to your question, but re: monitor arms.. mine can be pulled out far enough it would touch my face. It mounts into a grommet drilled into my desk. I assume there's other reasons this isn't workable for you, but if it's for lack of finding a suitable arm, let me know and I'll find a link for you.
My other recommendation would be to consider a standing desk. Even if you prefer to use it sitting, you can tweak the desktop height to your liking and help mitigate the posture issue.
Another option: you can buy a $40 monitor arm (they're all pretty good in my experience) on Amazon and mount it in the front of the desk to the left or right side and then swing it into position in the middle anywhere, even feet in front of the desk.