the original Deck is actually the best device you can get for Moonlight / Sunshine streaming from a performance point of view. It supports AV1 decoding and it’s by far the best SoC for consistent, frame paced streaming.
5 GHz Wi-Fi is perfectly capable to stream at 4K/60 as long as your host is wired.
Steam Deck’s HDR is only supported on Vulkan, not EGL, so until the Moonlight devs write a bespoke Vulkan renderer, you won’t be able to use it.
Are you sure Steam’s Remote Play is able to stream in HDR? As far as I’m aware it’s SDR only.