Which diet for Itchy dog with allergies?

My pups brother has multiple allergies which cause runny stools and lots of itching. He has been prescribed Apoquel, but she has since found out that it has some serious health risks. The vet didn’t mention this, nor did he say it was only to be used short term until a more permanent solution could be found.
He’s currently on “Gentle” Goat kibble.
Does anyone have any advice please?

There are a couple of routes you can go.

The first is allergy testing to find out what the dog is allergic and intolerant to and eliminate these from the diet.

The other way is to go down the route of an elimination diet. Something like Wafcol Salmon and Potato usually eliminates all of the current food/protein sources. If the problem doesn't improve after one month, then it's not food related. If it does, add one food at a time so you can make your own list of what to avoid

What breed is the dog? Sometimes it can be anything to do with poultry. As Chris say's elimination is the key. But If it were me I would just try chicken for a couple of days and if that gives no improvement then you will know that it is that. My Collie was allergic to anything chickeny. So was on Salmon and Potato which suited him really well. Another one to try is pork and potato. But check the ingredients because some of the ingredients is coated in poultry fat. I also found that rice causes problems.