I use a foam sprayer attachment for my pressure washer. I've never used one that works with just a water hose ( although I know Chemicals Guys sells one), but I suspect I wouldn't like it because I didn't like mine when my good pressure wahser broke down and I had to use it on cheap electric...
I'm also contemplating if the Yakima is the best option.
I think the iKamper Skycamp 3.0 Rooftop Tent might be better, but wonder if it'll be an issue if it sticks out past the bed about 8".
If the phone key works via Bluetooth, can't you simply turn off your phone Bluetooth for now while you're working in your garage 'til Rivian fixes this issue?
I read somewhere that with the underbody shield, the entire weight of the truck (7K lbs) can be supported on 1 square inch area or so under the truck. I don't know how that compares to the R1T without the shield.