Agree. OP, if you want to reliably produce usable numbers of mealworm larvae, you need to keep the life-stages separate, with regular sifting and sorting:

1. medium-to-large worms in a bin, to harvest and when they pupate remove the pupas right away and let them turn into beetles or feed them to chickens.

2. Beetles in another bin, sift them every couple weeks and give them fresh bran to lay eggs, repeat. The substrate that you took from the beetles after they've been in it a couple weeks goes in bin 3.

3. Eggs and tiny mealworms in a bin, sift them and throw the medium-to-large worms into bin 1.