Our dog has a sensitive stomach, too, so we keep him on science diet advanced fitness small bites.

https://www.chewy.com/forza10-sensitive-digestion-grain/dp/136979

I have a beagle who has suffered from diarrhea ever since we got him in February. This food contains anchovies and it does smell, but it definitely helped firm up his poop. We still give him Fortiflora daily, but nothing else from the vet helped (prescription bland food, medication, etc.) and testing turned up nothing.

The downside is that it's pricey.

Another vote for Merrick

Merrick.

We are using Blue Wilderness--our dogs really like it.

For our lab with a sensitive stomach, we feed Taste of the Wild salmon. He does really well with that. No more vomiting.

Fromm heartland gold.
Grain-free, made without chicken. Apparently, chicken in previous food was the reason for stomach issues.

I would get the dog checked by the vet to be sure it's really a food issue. If it is food, you'll just have to keep trying till you find one that works. We feed our German Shepherds Taste of the Wild.

Kirkland (Costco) grain free.

We had to go with a vet nutritionist who prescribed a home cooked diet for our sensitive dog. It's not for the faint of heart in that it means cooking, weighing all ingredients, mixing the daily amount to potion out over the day, etc. However, for our dog, it has been miraculous. I was trying to make my own home formula for a time, but it turns out I wasn't feeding quite enough and was missing some nutrients. Our vet referred us to the Tufts vet nutrition center. Vet sent all records, then did a phone consult with Tufts vet, then I met with her by phone. Taking all issues and medical conditions into account, they then send a detailed recipe and instructions on how to introduce the food and monitor. Also checkd up by email for several months until fully stable on the new diet. For a lower fee, they can also find a commercially available diet that matches your dog's need if a home cooked diet isn't warranted. We had done single protein, unusual protein, vet prescribed formulas, everything, but nothing worked for very long. This has been the answer. And yes, I get really tired of cooking, measuring, making sure I have enough fresh ingredients. But then I see our dog thriving and playing again and I have to keep it up. Good luck sorting out your dog's needs.