This might help :)
I’m unsure about 12+ hours. In New Zealand, I’ve heard it advised that if there is ice crystal in the bag, it’s safe to re-freeze as it has stayed at a low temperature. I’m not sure if this is scientifically based and I wouldn’t want to keep it for a year after partially melting, but for your holiday until it’s used, I think would be fine. I would put it in a cool box/bag with lots of ice packs compared to the amount of breast milk, then wrap the cool bag in blankets/clothes and place in the middle of another bag?
https://movemamamove.com/relocation/how-to-keep-breastmilk-frozen-while-traveling/