The styloid removal can help some w/ posture as muscles that are being “held hostage” i.e. are chronically tense/active by nerve irritation may begin to relax on their own post op. The doctor who told you the lack of a neck curve is irreversible is WRONG (unless your cervical vertebrae have been fused)! There are many YouTube videos done by chiropractors & PTs who demonstrate exercises you can do to begin restoring the lordotic curve in your neck which in turn will help your posture. It’s a long, slow process i.e. takes a year or so to make decent progress, but if you’re diligent w/ the neck exercises, which you must do gently!!, then you will make progress.
As @Jules mentioned, if you use the magnifying glass search tool, & type military neck (or forward head posture or Upper Crossed Syndrome - all names for basically the same problem) into the search box, many posts will come up on these topics.