They’re a year old now and started laying before ‘winter’. We do not get much of one in central Florida.
Chickens usually slow down or stop laying in winter because the days get shorter, not because of cold weather. Even in Florida, you would have shorter days in the winter. So that is one possible cause of the egg shortage.
But since this is now May, I would certainly expect them to have started laying better again-- probably starting several months ago!
They were laying about two eggs a day between the four of them on average. Now we’ve gone months with barely one a day. A lot of days we get none. I had one hen go broody back in February. I finally gave her some eggs to sit on and those chicks are about a month old so I imagine she’ll begin laying again soon.
Is there any chance they are laying in a hidden nest? This is much more common if they are let out to free range, or have a large fenced run, and much less common in smaller areas. But even in small coops, they can occasionally hide nests in a corner of the floor, with bedding kicked on top.
Maybe one or more of them are eating eggs. Or something else is coming in to steal/eat the eggs (person, dog, rat, snake, etc.)
Do you have any other adult chickens? Sometimes a hen is "not laying" because she lays a different color egg than the person expected, so a different hen gets credit for it. (This is most common with Easter Eggers, if they lay brown eggs instead of the expected blue or green, but it can happen with other breeds/colors/sizes of eggs too. I once thought I had a particular big hen laying, but it turned out to be a bantam hen laying bigger eggs than I expected.)
You can check their butts to see if they are laying.
https://www.backyardchickens.com/articles/who-is-laying-and-who-is-not-butt-check.73309/
Look at one of your pullets (the 9-weekers), or at the hen who was broody, for an example of a not-layer. Then check the other adult hens, and see if you spot differences. After seeing both versions, I have found the difference to be quite obvious. The vent looks different (like it could stretch enough to let an egg go through.) There are also bones below the vent, one on each side. In a not-layer, their tips are easy to feel through the skin, close together and right below the vent. On a laying hen, the tips are still easy to feel, but they are further apart and lower down. Again, it can be pretty obvious once you've felt both versions.
If they all appear to be laying, you have one kind of problem (where are the eggs going?)
If some of them are not laying, you have a different problem (why are they not laying?)
I am guessing they are laying, but something is happening to the eggs. But that is a guess, and I could be wrong.
I’ve read that an all flock feed shouldn’t be fed to the younger chicks
I thought "all flock" was specifically meant to be fed to all ages

You can read the label on the back of the bag to see, both feeding directions and nutrient information.
To be suitable for chicks:
Protein should be about 18% or more.
Calcium should be less than 2%
If it is a complete feed for chickens (directions say "feed as a complete ration"), the protein and calcium levels are acceptable, and the pieces are small enough for the chicks to eat, it should be fine for chicks, no matter what name the feed company chooses to call it by.