It is known that King K Rool can score combos after using Down Throw, but this is dependent on the opponent's damage percentage and mashing ability. There is a list of kill confirms from Down Throw + Up Tilt and Down Throw + Forward Tilt, but not one for true combos (inescapable combos regardless of what your opponent does). For Up Tilt and Forward Tilt, they can be true combos at earlier percentages.
Assuming your opponent is perfect at mashing, what true combos can follow King K Rool's Down Throw, and at what percentages? Note that mashing well can greatly reduce the time buried even with >150% - here's an example.
Down Throw + Up Smash can also be used against opponents who mash, but it relies on the opponent unburying to hit it with Up Smash. Down Throw + Up Special cannot form a true combo.
60 + .75 * (opponent%)frames. Perfect mashing would be one input per frame that reduces the bury by 15 frames, so divide that formula by 15 essentially. Mean bury time with perfect mashing will be4 + .05 * (opponent%)frames. Again, I don't have a switch to test FAF on down throw, but if you have that then the math for whether any move is true at any percent becomes fairly trivial.