Most of the external portable screen projects I’ve seen for the deck use devices that can directly receive video over usb-c, like XR glasses or portable screens. I don’t know of a good way to convert video over usb-c (or hdmi from a dock) into something that your Android tablet can receive and play.

The easiest thing to try next would be to install Sunshine on your deck, and Moonlight on your tablet. It will still streamed over wifi, but a lot of people get better stream quality/less lag than with steam streaming.