Only Matthew says there was a guard on the tomb, there is not mention of any guard in any other gospel and common sense would make one wonder why the Romans would post a sentry on a Jewish tomb of what amounted to a simple commoner executed for sedition against the Jewish state not the Roman empire?
other contradictory details include

Mark 16:1-3, Luke 23:55-56—A group of women who were at Jesus’ burial come back later to anoint his body
Matthew—Joseph wraps the body, and the women come the next morning, but no mention is made of anointing Jesus
John 19:39-40—Joseph of Arimathea anoints Jesus’ body before burial

Mark 16:1—Three women visit Jesus’ tomb: Mary Magdalene, a second Mary, and Salome. They arrive after sunrise
Matthew 28:1—Two women visit Jesus’ tomb: Mary Magdalene and another Mary. They arrive at about dawn
Luke 24:10—At least five women visit Jesus’ tomb: Mary Magdalene, Mary the mother of James, Joanna, and “other women.” It is early dawn when they arrive
John 20:1—One woman visits Jesus’ tomb: Mary Magdalene. She later fetches Peter and another disciple.
It is dark when they arrive

Mark 16:4, Luke 24:2, John 20:1—The stone in front of Jesus’ tomb had been rolled away
Matthew 28:1-2—The stone in front of Jesus’ tomb was still in place and would be rolled away later

Mark 16:5—The women enter the tomb and meet one young man in there
Matthew 28:2—An angel arrives during an earthquake and rolls the stone away, and sits on it outside. Pilate’s guards are also there
Luke 24:2-4—The women enter the tomb, and two men suddenly appear — it’s not clear if they are inside or outside
John 20:12—The women do not enter the tomb, but there are two angels sitting inside

and all this culminates in
Mark 16:8—The women keep quiet, despite being told to spread the word
Matthew 28:8—The women go tell the disciples
Luke 24:9—The women tell “the eleven and to all the rest.”
John 20:10-11— Mary stays to cry while the two disciples just go home

Even as a child this all seemed like unreliable bull to me.

You may be overthinking it! It’s a story. Anything is allowed to happen.

Very true, but there is little chance that any official records any court would have survived. We have very few records of any Roman court cases or matters of military affairs. Even 99.999 percent of the Roman worlds high literature has gone, thanks in part, to Christians who burned and destroyed vast amounts of it, except for a few famous books which got passed down, in the main part thanks to the Islamic world, which unlike many Christians, did at least value literature.

Which is one of the things which make the gospels so doubtful. That not one, but four, strange books should suddenly appear, many decades later than the events they claim to describe, and all claiming to be first hand accounts, though they hardly agree on anything, is suspicious in the extreme.

The usual "thinking" in my observation and experience is along the lines of "billions of Christians can't be wrong". It is just group inertia. It falls apart when you understand it's just a campfire story.

There would have been no reason to guard a dead guy no matter what lame reasons theist come up with. The story was simply made up.