What a stage actually is
A pipeline stage is a checkpoint that a deal passes when something verifiable has happened. That is the whole definition, and almost every broken pipeline breaks by ignoring it. Once stages start meaning how confident the salesperson feels, the pipeline stops being a record and becomes a mood board that happens to have a currency total at the bottom.
The test is simple. Take five open deals, ask three people to place each one independently, and compare. If they disagree on more than one deal, the stage definitions are the problem, not the people.