Hello and welcome to the forum. You and your dog seem to be having a rough ride right now. Problems like this are never easy to solve. We automatically assume that it is food related and in some cases it is but there are other issues that might be at play such as environmental allergies or maybe immune system problems.
With regard to your enquiry about anal glands, yes they can most certainly cause itchiness and with great intensity. However, they don't cause allergies as such. It is thought that fibre is helpful to bulk out the stool which then compresses the glands and empties them as it passes through the anus. Some dogs are at risk of anal gland problems but I don't fully understand why, unless it is because of the anatomical position of the glands themselves. I had a rescue dog that needed to have them emptied by the vet every six to eight weeks - nothing wrong with her poos. Fibre is thought to be helpful in preventing impact ion of the anal glands.
Looking at the Arden Grange Sensitive review here
I see that it has potato as the first ingredient. When selecting a dog food, it is always advisable to look for one with the the primary meat source at the top of the list of ingredients. There is a lot of potato in it so it has a correspondingly high carbohydrate. I can't say whether your dog has food intolerance and to be absolutely sure you need to do a proper elimination test under veterinary supervision. Sometimes folk go off and buy something else, then if that doesn't work, they try a different product and so it goes on. That is why you need to change food in a structured way and with care. The suggestion of your trainer to try a raw diet is helpful but if you do not want to do this then it is best not to attempt it. There are two methods - raw complete meals and making up a balanced meal yourself. The former is easy - no different to feeding canned dog food. However, it can be a bit pricey, especially with a large dog such as yours and I am not sure whether it could be fitted into your budget - you would have to check. The second is probably cheaper but requires a bit more input from the owner.
The other thing that occurs to me is coat care. You have a mixed breed dog with ancestors who had different coat types. Have you spoken to a good, experienced groomer about this? It may be helpful to use a good quality shampoo at intervals. I use Sebocalm because it is soapless but your vet may have a better suggestion in view of the yeast infections. I have heard that Neem is good for such conditions. Remember that any dog that is scratching runs a high risk of infecting the skin and this leads to more scratching. In view of this, antibiotics for at least one week can often be useful. Another thing that might help is to ensure that your dog is thoroughly and regularly groomed to get out the dead hair. I don't know the coat type of your dog but choose a good quality brush, steel comb and possibly a furminator. If you have a qualified groomer nearby, maybe it would be useful to book your dog in for a bath and groom and to have a chat with her or him.
On the subject of changing food, I can't really suggest any one product because there are so many. Also, I do not know what (if anything) your dog is intolerant of. Of course you can use the filters in the Dog Food Directory of this website - the instructions are scattered about this forum. It would help you to narrow down the choices. Ordinarily, it is the wet foods that tend to be simpler in formula than dried food and there are some good quality ones these days, rating 4-5 stars but they may be a bit more costly. If you wish to stay with dry food then I would suggest that you look for something in the 80:20 range, grain free. There are quite a few around, and you can easily find these by setting the filters in the Dog Food Directory to dry food/grain free and 4 to 5 stars (that one is a slider). They tend to use sweet potato as the carbohydrate source and this is thought to be a better option than potato. You can also filter out yeast under 'Avoid Ingredients'.
With high quality foods you need to be very careful not to overfeed and the food must be
weighed. They will probably be more expensive but the quantity that is needed for good health and weight control is often much smaller than lesser quality products. Some examples are Eden, Evolution, Millie's Wolfheart, Orijen but there are plenty more.