The R1T's in the Long Way Up were using 33" 275/60R20 Pirelli AT tires (see photo). Rivian went with 34" 275/65R20 tires after that. Maybe the change was based on data from that trip.
It sounds like you went with an LT (Light Truck) tire, which is why it's heavier at 53 lbs (vs 48 lbs for the Rivian AT tire). Did you consider going down a size to 275/60R20 (which matches the 33" diameter of the 21" wheels)? The Defender tire in that size is 42 lbs, the Scorpion AS+ 3 is 38 lbs.
I had asked CS a while back about the size of the spare for the R1S. Here's what they said: "Regarding the compact spare, the wheel size is standard, no matter your wheel configuration. More than anything, it’s designed to be used temporarily until you can get a proper wheel replacement. "
Same here. CS told me today that "we’re on track for the March-April delivery timing". 2018 R1S LE, Rivian Blue, Ocean Coast, 20" bright wheels. I haven't heard from a guide though.