Isolated AC/DC Buck Converter
There are lots of buck-derived isolated topologies (Google it). Nothing stops you from using a buck after an isolation transformer, but then the isolation transformer is 50 or 60 Hz (i.e. big). The low duty cycle might be an issue as well if you insist on a 1:1 transformer. A buck-derived isolated topology can use a high frequency transformer and will be a much smaller solution. In the example you provide a flyback might be a better solution.