The 64GB emmc drive in the Deck uses the m.2 expansion slot, so you can pop out the emmc drive and swap in a 2230 form factor nvme drive and be gtg after reinstalling SteamOS.
I myself went from a 256GB drive to a 1TB drive, and this weekend, I'll be popping in a 2TB drive I just received in the mail.
Couldn't agree more. That knobhead looks to Elon for inspiration on how to run a business.