In the original Hebrew, “weapon” (keli) means any tool made for harm. The second half of the verse names the main one: every tongue that rises against you in judgment you shall condemn. So physical arms are covered, but the emphasis is on accusation and slander. Words.
The promise was spoken to Israel coming out of Babylonian exile, and the verse ends by calling it the heritage of the servants of the Lord.
“Shall not prosper” doesn’t mean it shall never wound. Scripture is full of God’s servants who suffered terribly. Stephen was stoned to death. The weapon did its damage, but it didn’t end God’s purposes in him or silence the church. That’s the scale the promise works on.
Some years my congregation reads Isaiah 54 during Eastertide, and I think that placement is the best commentary on it. The cross was the most successful weapon ever formed against God’s purposes, right up until Sunday morning. Now I hear it as nothing that happens to you gets the last word. The invincibility version falls apart by breakfast, like you said.