This!
And you have to be prepared to take a leap here. There’s fungal agents in everything, in the air we breathe, in the food we eat. Unless you’ve found research
online that specifically says DED is detrimental to felines/animals in general, just go with whatever slippery elm powder you find, or omit it entirely. And consult with your vet, not us tree dorks. Cats are not trees, the fungal agent that causes DED in an elm tree is not likely to do any harm to your cat.
And be prepared to let go. You can only do so much.
We routinely work on trees infected with this and that, that have been sprayed with insecticide, fungicide, what have you, inhaling fine dust particles when we make our cuts. To no known detriment to our health. (Aside from allergies one might have to a species, and to creosote treated wood)
The fact there is no research on the subject of humans inhaling fine dust particles of DED infected trees should give you a sense of what kind of information you’ll find specific to felines.