Armor material is entirely up to you. I have 3mm Sintra for all of my armor (except knees) and it's held up for 50+ troops with no problems. 6mm is a bit more durable though. I'd stay away from 3D printing your collar and chest plates though. Shaping and trimming will be really difficult unless you can get both of those things right straight from the printer.
Things like gauntlets and knees do well being printed though. Same with helmets. I think resin or 3D print would do just fine for your bucket. Sintra, ABS and metal are typically what we see being used for a majority of the armor, though.
I don't have much suggestions on the animal skull paint, but I will suggest skipping the shading on your actual armor. Being a large, moving, 3D object, you're going to have natural shading depending on your lighting. To artificially add it might be weird.
However, I highly recommend adding depth to your weathering!
Also, the marbling idea sounds neat but I'm not sure how that would look on a large scale. If you could do a practice or a test piece, that might help for us to get a visual!