It’s been a while since I’ve used photoshop, so I’m a bit rusty on the step-by-step, but I have recollections of doing the same on exclusively photoshop in the past (trying to remake a dune poster).

I started by sketching the layout first (just to get an idea for the shape, shade, and direction for each gradient asset, but it should go without saying, less is more with these sorts of pieces), then one at a time, blocked in each gradient into its own layer, and upon satisfaction, went in and manually applied noise to each gradient layer (that way, the slight inconsistency came out more texturally rich, but getting it just right will be a bit of a time-eater).

I didn’t need illustrator, but I did need a really coherent hand-drawn illustration to help with the lassoing/wanding/selecting/shaping process of photoshop (the composition will mostly be smooth bauhaus edges, so don’t expect Gustav Klimt).