Word by word, I’d translate “I raised a monument more long-lasting than bronze, in a royal site more high than pyramids.”
And a question, why pyramidum is in the plural genitive case, considering that altius is the neuter form of comparative of superiority which accompanies the neuter substantive in ablative case, situ?
The accusative altius in on a par with perennius, agreeing with monumentum, and the ablative regali situ is on a par with aere, governed by the comparative adjective: “longer-lasting than bronze and loftier than the royal situs (decaying structure) of the pyramids.”
He’s making a grandiose claim for his poetic achievement in the three books of the Odes. (He added a fourth later.)