Early Booking/Placeholder Question

Help, please! I would love to book an Alaskan cruise for May 2027. I am silver so I can book today. The price is already high for a verandah room, which i have in my cart.

We are sailing on the Treasure May 2nd 2026. I was planning to buy a placeholder.

Should I still book the 2027 cruise today?

Can I apply placeholder/switch to Costco after my May 2026 cruise? New to this so any tips really appreciated!! Thx!