Hello and welcome to the forum. I am sorry to hear of the problems that you are having with your dog. It sounds as if she is intolerant to something but without specific testing or a lengthy exclusion diet it is difficult to tell what that may be. By now you may be getting a feeling yourself as to what is causing the loose poo/diarrhoea.
All dried foods tend to have a lot of ingredients so it makes the problem of finding out what the dog cannot cope with more complex. The advice usually given (which is what you have already done) is to exclude cereal and wheat first. The second most common intolerance is to certain types of protein, chicken being fairly common.
I may be wrong but I think your idea of trying her on a wet diet might be the way to go. They tend to have less ingredients and are easier for the dog to digest. I take your point about the expense but if it helps it might be cheaper in the long run because of the repeated visits to the vet. If you go over to the Naturediet website here
there are numerous fact sheets on this sort of problem. They do a sensitive version and I think that is what they usually suggest as a starting point. However, all their products contain white rice so if you suspect that your dog has problems with this then you need to be thinking about a wet food that is grain free. Using the filters on the Dog Food Directory of this website should help you find one. Wainwright's (Pets at Home) do a grain free wet version that scores very highly on here. If you go down this route then it would be best to stick with one flavour for a week or so to see what the dog's response is.
The only other thing I can think of is to try raw but unless you go for the complete meals such as Nutriment, Natures Menu and Natural Instinct, you would need to learn a bit about it because you don't want the diet to be lacking in essential vitamins etc. If you decide to try a raw diet don't transition, just stop the dried food and start feeding the meat. Also, if you do try one of the aforementioned products, it might be best to call their helpline to get further advice.
I hope that you find a solution to this. Please would you keep us updated?