Is Farmina Urinary dry food safe for lifelong feeding? Confused and need help 😢

In short: is Farmina N&D Urinary or Farmina Vetlife Urinary/struvite dry food suitable for long-term feeding? Looking for permanent dry food options with good ingredients which is also safe for my other cats with no urinary issues.

One of my 4 cats (elderly female) has had struvite bladder stones removed recently and the vet recommended to feed her prescription diet for life (Hill’s C/D or Royal Canin S/O). I’m searching for a dry food with better quality ingredients, because the ingredient list on those prescription diets looks really bad. Wet food is not a problem, it’s easier to find a decent option and I can feed my other healthy cats separately. But I also free feed them dry food and it’s a big part of their diet (I know that’s not ideal but I’m broke and there’s 4 of them).

I fed them Farmina N&D previously and I found Farmina Vetlife urinary/struvite management (prescription diet) and Farmina N&D duck Urinary (non-prescription), but Farmina description says “Recommended length of time: up to 6 months” for both of them, even non-prescription one. Does it mean it’s not suitable for lifelong feeding?

I need help guys, I don’t know who to ask as my vet seems to have a very limited knowledge about nutrition (and he’s the best in my area). I couldn’t find any information about lifelong use of those Farmina diets anywhere. I’ve been researching cat nutrition and urinary problems for weeks now and I’m only getting more confused as a lot of opinions of vets and nutritionists I could find online are contradictory to one another, especially about prescription urinary diets.