Getting the gold might have been Cersei's main objective in assaulting Highgarden. Makes sense that they'd retrieve that, load it up into the wagon and then rush that ahead, back to King's Landing to settle all debts and pay off the bank. It probably had an entourage of its own, but they wouldn't assign the whole army to protect it when the requirement was haste.
Then you have the bulk of the army - it was probably an easy victory over the Tyrell forces but they've just won a significant victory for their queen, and you also have dozens if not hundreds of other wagons whose job was to gather crops and supplies from the Tyrell territory. Makes more sense to keep the army around for this, especially if there's likely to be any resistance from the citizens or anyone still loyal to the Tyrells. Then, once that is done the army escorts the caravan back to King's Landing, predominantly in a rearguard formation which is when Dany attacked. This makes sense to me and I don't feel like it's contrived or unrealistic.