PCB and plates thickness and material questions. #11

Hello,
Thanks for sharing this awesome project.
I'm currently in process of ordering pcbs and plates from jlcpcb to build taira keyboard. I've stumbled upon several very basic questions I couldn't find answers for:

• Is default 1.6mm a good thickness for both pcbs and top/bottom plates?
• Could I order top and bottom plates in aluminum? Is it a good idea in the first place?
• BOM mentions Spacer M2 4mm, but according parts link is pointing at 2x3mm spacer, am i missing something?

Thanks again.