I am gold (one cruise away from platinum) and its pretty easy to get everything except Palo Brunch which i did not get on my last two cruises.
However, i could have gotten it if i would have chosen to stay on the ship during a port day or dropped a remy brunch and i didnt want to do that. If you want more chances of getting things, go for a brunch or onboard activities on port days. Everyone wants things on sea days.
Most excursions don't actually sell out, minus the brunches, and you can easily book onboard. I personally NEVER book port excursions thru Disney from my experiences (Cancellation policy is horrendous/ non existent without paying more than the excursion itself to see an onboard doctor and prove that youre sick) but if I was feeling brave lol I would not book until I was onboard.
They have a limited number of reservations showing online and then once onboard MOST things are available. Its kind of like DVC in that getting highly sought after rooms (in this case, cabanas, massages on
castaway cay, palo brunches, royal princess gatherings) is going to take some time out of your day to book as soon as possible and get up earlier or stay up later to accompish (time zone dependent).
The first day of cruising for me is spent speaking to dining, the spa, etc just adding on activities. Then once everything is sorted, hit the buffets!
Even platinum won't prevent you from having to do stuff like this.for.some things. First dibs goes to $$$$ concierge and even if you've sailed on 10 cruises if you want a cabana good luck. You better be really lucky with your stalking or concierge.