The creed only lists appearances and says nothing about an empty tomb. Couldn’t the earliest tradition have been about visions rather than a physical resurrection?

The creed’s sequence is died, buried, raised. To a first-century Jew that sequence meant that what was buried is what was raised, which is a body, not a vision; there was no other meaning of resurrection available. Paul’s argument later in the same chapter about what kind of body is raised only makes sense if he believed the tomb was empty. The fuller version of this objection, that the reports were visionary and the physical details grew later, is treated as its own rival explanation in the Resurrection of Christ case.