Make the bumbu. Blend the shallots, garlic, chilies, macadamia nuts, turmeric, and ginger with a splash of water into a coarse paste. Heat 2 tbsp oil in a small pan over medium heat and fry the paste, stirring often, for 6 to 8 minutes until it darkens a shade and the oil starts to separate at the edges. This step cooks out the raw sharpness and is what makes the glaze taste deep rather than harsh.
Finish the glaze. Add the 4 tbsp kecap manis, the tamarind paste, and the palm sugar to the fried paste. Simmer for 2 to 3 minutes until it thickens into a glossy, spoonable glaze. Take it off the heat and split it into two bowls, roughly half for marinating and half for basting.
Prep the fish. Pat the fillets dry and score the thick side two or three times down to the skin. Rub them all over with the salt and the marinade half of the glaze, working it into the cuts. Let them sit for 15 minutes while you make the dipping sauce.
Mix the sambal kecap. Stir the 3 tbsp kecap manis with the lime juice, sliced shallot, and bird's eye chilies. Set it aside so the flavors mingle while the fish marinates.
Cook the fish. Heat a cast iron pan, grill pan, or outdoor grill over medium-high and brush the surface with the 2 tbsp oil. Cook the fillets skin side down first for 4 to 5 minutes without moving them, until the skin is crisp and releases cleanly. Flip, then baste with the reserved glaze in thin layers, cooking another 3 to 4 minutes until the fish flakes and the glaze has charred in spots. Under a broiler, cook skin side down 5 minutes, then flip, baste, and broil 3 to 4 minutes more.
Serve. Move the fish to a platter, scatter with fried shallots, and add the cucumber and lime wedges. Serve hot with the sambal kecap for spooning over, and steamed rice on the side.