Dill and Fennel plants are turning brown and dying - need help!! [central Texas]

So I transplanted some dill and fennel plants into my garden and for the first month or so they have been doing great. But recently, almost all of them have been suffering. The fronds on many of the plants are turning brown and wilting, and some of the plants have already died entirely. A couple of the plants haven't turned brown but have still wilted and nearly died. The plants that look brown and wilted also have a bunch of these small, flying, dark-bodied insects all over them - not sure if those are part of the problem or are just attracted to the dying plants for some reason.

The only two changes that have happened recently were some heavy continuous rains, and I also applied a thin layer of cedar mulch to the beds. Some of my brassicas also started to look a bit sad and wilty and yellowing around the same time, but not nearly as bad as the dills and fennels. I recently read that cedar mulch can be allelopathic so today I pulled all of the mulch off of the beds in case that was a contributing factor. After all the rains, I think the soil was fairly waterlogged, and it even still feels pretty wet today even though it hasn't rained for some days. Which makes me think that overwatering is maybe the main issue, but not sure. Any advice? Is there any way I can save these plants? Thanks!!

Cedar mulch can be a problem; it can have allopathic substances in it.

Two more things to think about; has it been getting cooler in your area? Maybe the plants are dying off in response to the fall. Or, have the plants set seed? In which case, all the energy is transferring into the seed. I would assume that there is some combination of these two things going on as Fall moves forward. Dill and at least some fennel plants are annuals, so they will not keep going once the fall comes, temperatures cool, and daylight shortens.

In my garden it's grows through the winter and sets seed only in the spring. It self seeds in my garden annual and doesn't even sprout until the cooler weather of fall. I'm also in central Texas so I'd expect similar timing.

Considering it does so well in my garden, I can't say what problems it may have. That's actually why I hadn't chimed in. I'm at the base of the hill country where the soils start shifting to pure caliche.

If it is an issue of drainage, it's not to late to replace dill. Mix organic matter (compost or aged manure is perfect for this purpose) at least six inches deep into the soil. Use enough to raise the bed surface by at least a few inches. Then either spread fresh seed or transplant seedlings into the bed. The plants will remain small over the winter (you may want to cover during freezes) but as soon as the weather starts to warm in the spring they'll rocket up.

Don't forget to mulch as the plants grow. If you can maintain a good layer of mulch, it will protect and feed the soil biology. That is what will develop a good soil structure in the future. Just mulching alone can completely change a garden bed over time. Ideally you only need to dig a well mulched garden when harvesting roots.