I commute via East Croydon into London Bridge. I usually arrive into LBG between 07:30 and 07:45 in the morning, and depart from LBG between 18:30 and 19:00 in the evening. This is probably not quite the times you will be travelling, however I will offer my observations.
Things seem to have stabilised at LBG following the culling of the West Croydon stoppers and some timetable tweaks. At the times I travel, you can usually get a seat if you are prepared to walk to the country end (which is the end with the main ticket office at East Croydon), and avoid the long-distance trains to/from the south coast. Brighton trains are particularly busy. Avoid Brighton, Bognor Regis, Littlehampton etc trains, and aim for Uckfield, East Grinstead, Horsham or Caterham/Tattenham Corner trains.