I am currently playing in Steam using my PSN characters transferred through cross-save. I want to buy the Forsaken DLC now while still continue playing in Steam with my old characters. Does this mean I should buy the Forsaken DLC through Steam or through PSN?
Purchase the DLC on the platform you wish to play on. If you wish to play on PC and don't plan on going back to the PS4, then purchase it on PC. Where your Cross-Save characters are from doesn't matter.