I'll go through my standard list.
The usual reason a flock stops laying or almost stops laying is the molt. I know, I know. It is spring where you are and many people believe that nonsense that chickens only molt in the fall. Most do molt in the fall but chickens can molt any time of the year. Have they been getting light to lengthen their apparent day and that light is now gone? That may be lights you added to get them to lay during the winter or a security light or street light that went out. That happens a lot.
Stress can cause a flock to molt out of season. Stress could be from predators, a change in the pecking order, or a change in housing to name a few. The question here is "Are you seeing feathers flying around?" If you are seeing feathers they are molting and not laying.
Sometimes stress will cause a drop in egg production without causing a molt. Severe weather, a perceived predator attack, or changes can cause them to temporarily stop laying. You mentioned some but those seems too long ago to be your current problem unless it is a molt.
I know you said it can't be, but a very common cause of them appearing to not lay is that they are hiding a nest. They can be really good at that, even locked in a run. But by now there should be enough eggs stacked up that you should be able to find it.
Another common cause of you thinking they are not laying is that something is getting the eggs. Many critters that take the eggs leave signs, broken eggshells or wet spots. Some critters in North America that do not leave signs are snakes, canines, and humans. Even if snakes are active where you are it is not a snake. Too many eggs and too consistent. A snake typically eats some eggs and then hides while it digests them. Then, a few days later, it comes back for more. I had a 5' black rat snake eat 4 eggs out from under a broody hen and come back for more 3 days later. Ate four at a time. Nope, not a snake.
I'm not sure what continent you are on so you may have different predators, but a fox or coyote would more likely have an interest in eating a hen than the eggs. Not likely one of them. Does a dog have access? They can swallow eggs whole and not bother the hens.
A human does not always mean a stranger. Usually it is someone you know, especially if it is consistent. People do these things as a practical joke or maybe they are selling the eggs somewhere.
Some diseases can cause hens to stop laying but if that were the case they'd be acting sick. And they can lay deformed eggs. I'll mention it because it can happen but if they are acting normal it is not likely to be a disease.
I don't know why you are not getting any eggs. With that many hens that age that laid through the winter you should be doing better. If yours were out ranging I'd suggest locking them up for a few days to see if you were keeping them from hidden nests or maybe locking out an egg eating predator. That's probably not helpful for yours. I will suggest discreetly marking a couple of eggs and leave them down there. If they disappear you will know something is going on.
Sometimes we can figure out what causes these things, sometimes not. It can be really frustrating. I wish you good luck.