Well I guess one way to look at is you're burning an additional 30K miles (so $300 worth in this case at the pay with miles rate, about $450-$600 at $0.015-$0.02 per mile valuation) for ~15K MQMs. I have to imagine finding a mileage run for 15K MQMs for $300-$600 would be tough but I'd also evaluate that based on my mileage balance and if I was saving the miles for anything else.
As for the A350 vs 767, I don't have much input there without any insight to seat availability. The A350 in Y is probably more miserable in Y than the 767 is in C+ unless you get row 30 or 41A/J or a seat in Row 40 on the A350. Routing via SEA both ways would about 1400 extra MQMs but on the flip side, that transcon across the US can feel brutal, especially west-bound.