Whisk pear juice, soy sauce, vinegar, vegetable oil, garlic, sesame oil, ginger, gochujang, brown sugar, and salt in a medium bowl. Pour 1 cup marinade into a large resealable bag or bowl; reserve remaining marinade. Whisk in gochujang, brown sugar, and salt. Add steak and toss to coat, then let rest at room temperature at least 1 hour. Heat oil in a large skillet over medium-high. Remove steak from marinade and pat dry; discard marinade. Cook steak until an instant-read thermometer inserted into the center registers 125°F, 23 minutes per side for medium rare. Transfer to a cutting board and let rest 10 minutes before slicing against the grain. Toss cabbage, 1 Tbsp. marinade, and 1 Tbsp. vinegar in a small bowl. Toss carrots, 1 Tbsp. marinade, and remaining 1 Tbsp. vinegar in another small bowl. Let rest at least 10 minutes. Toss spinach, grains, and salt with 1/2 cup marinade in a large bowl. Divide among 4 bowls. Top with beef, cabbage, carrots, peas, and kimchi, if using. Place 1 fried egg in the center of each bowl, if using. Top with scallion and sesame seeds. Serve with gochujang and remaining marinade alongside. Steak can be marinated 10 hours ahead; chill in bag. Let steak come to room temperature at least 1 hour before cooking. Cabbage and carrots can be marinated 1 day ahead; cover and chill.