Pricing for August 2024 will not be set for many places yet. And as a practical matter, most of Europe's beaches -- and likely those that meet your criteria - are in the Mediterranean. Regardless, August remains the most popular month for Europeans to travel, so you may have to adjust your expectations of a beach destination with few people.
As for specific destinations, you may be best off doing some research in the Luxury Hotel forums or narrowing it down yourself a bit. A remote luxury hotel that isn't too expensive or crowded in August somewhere on the beach in Europe is a hard ask