Thanks @MKKL !
The overall workflow has been pretty much the same for all the images.
I set up the overall shapes using the Creative Bundle add-on to carve the main geometry. Then I added the details using Random Flow (you can see a short time-lapse here).
Everything is rendered with Cycles-X, except for the volumetric in the first image, I used Eevee for that.
Lastly, I did the comp in Fusion, and Affinity Photo.