It's not a huge property, so personally I would say do no more than 5 days max and consider somewhere else. I'm also slightly wary that you highlighted the beach as your top priority: beaches are not great on Koh Samui in general, and by the Meridien in some seasons it's almost underwater. Maybe do 5 days here and 5 days at a cheaper property somewhere less spoiled (and less likely to be available on AMEX points) like Koh Lipe or Koh Kood. Krabi could be a good compromise, though it's not somewhere I'm best placed to advise on.

The restaurant on site at the Le Meridien cooks perfectly good food, though is definitely not a foodie destination. Having said that, you're on Koh Samui which caters all kinds of foodie needs and there are plenty of options within a 15-20min cab ride - though you're going to find more lobsters and wagyu steaks than you are sophisticated fine cooking of the sort you'd find in modern places in London or Paris.