Not on the shores of the river, but in the centre of the city near the shopping area, Aman Nai Lert opens in its Bangkok highrise on April 2, and is bookable now: https://www.aman.com/hotels/aman-nai-lert-bangkok

Most rooms are 94 to 98 square meters, low season rates are around 1400 Euro including taxes, fees and breakfast (it's a great progress that they now show the inclusive-price at their booking machine!), over 2000 Euro in October and near 3000 Euro in christmas high season.