Hear me out:

Annabeth does not want to do quests for College recommendation letters. Annabeth finds out about the Norse Pantheon from Magnus, some of them even seem to be far chiller than the Greek/Roman’s.

What I’m trying to say is the three of them should plan a Heist centred on breaking into Valhalla just to get Odin to write them a letter.

The epilogue could be Poseidon talking to Percy about who he’s going to ask and Percy’s just “oh some Norse Gods wrote some for us for free xoxo” and Percy and Annabeth get their letters from the Greeks on the basis of “Do Not Tell the Romans about the Norse, Please”