I'm officially an idiot, it was soo simple. See code below, hope it helps future idiots improve their coding skills.
private void InsertIntoBookmark(string bookmarkName, string text)
{
if (Document != null && Document.Bookmarks.Exists(bookmarkName))
{
var range = Document.Bookmarks[bookmarkName].Range;
Document.Bookmarks[bookmarkName].Delete();
range.Text = text;
// replace bookmark
Document.Bookmarks.Add(bookmarkName, range);
}
}