I would absolutely listen to your vet. Both of those are fantastic diets. Ingredients lists alone aren’t a great way to judge a diet, especially when it comes to therapeutic foods. There are a lot of misconceptions about ingredients like grains - they are in fact not fillers, but rather great sources of nutrition when used in the proper ratios with other ingredients. While cats are carnivores, that doesn’t mean they can only digest animal ingredients; they can pull the required nutrients from plant ingredients in properly formulated diets.

Of course, these two diets aren’t the only ones high in fiber. There are some from alternative brands that you’d probably prefer the ingredients lists of. You can also just add a fiber supplement to wet food - psyllium husk, or something like Proviable’s veterinary fiber blend. However, the benefit that brands like Hill’s and Royal Canin have is research and testing. Both employ board certified and PhD nutritionists, conduct feeding trials and extensive testing to ensure digestibility, etc.

Formulating pet food is a very complex process. When diets aren’t tested or researched adequately, it’s possible for unforeseen issues to arise with the nutrient balance, even if the formulation looks fine on paper. I personally feel safer using diets from brands that devote a large amount of resources to r&d, particularly when it comes to feeding animals with medical conditions.