I occasionally buy tuna for the cat I look after. But our local supermarket has tuna in spingwater (the other options are oil and brine which have bought before but rinsed off).

Yeah catfood products can be really shitty. As well as salt, cats are sensitive to sugar and can get diabetes. The cheapo catfood often has loads of sugar too. Although I did find one that was sugar-free and grain-free, saw it recommended on a cat forum for people struggling with prices of catfood. The expensive stuff is an insane price.

You can get taurine supplements, it seems pretty essential for cat eye and heart health in the little research I've done. No idea what the taurine levels are in tinned tuna or meats. (I think stuff like animal organs /offal is higher in taurine). Again , no expert lol I just did a bit of research when my flatmate rescued a poor wee thing from dying and she now lives with us:)