I'm not sure how you're going to drive I-15 from California to Washington, maybe you mean I-5. You'll have to go over the mountains in northern California on I-5. I'd be a bit concerned after reading the owner's manual for your 2019 Subie.
Here's one quote: SUBARU warranties do not apply to
vehicle damage or malfunction caused by
trailer towing.
Here's another quote that isn't encouraging: When towing a trailer on a long uphill grade continuously for over 5 miles (8 km) with an outside temperature of 104F (40C) or above Maximum total trailer weight 1,350 lbs (612 kg)