So, I'm a Xbox user that is gonna move to Steam with cross-save. My doubt is, for now, do we have to wait until new light is available so the game is free for the ones that didn't download it in the pass when it was indeed free? Or will we have to purchase the basic destiny to do so?
You will be able to play on Steam for free on October 1 when New Light is released. New Light will be released on all platforms supported by Destiny. You will only need to purchase DLC content not included with New Light on Steam if you wish to play that content there. That would include Forsaken, Forsaken Annual Pass, and Shadowkeep.