I have an “anti-yellowing” soft sides and hard back case and it was fine for about a year, almost no yellowing at all (just a slightly tint after like 8 months) unfortunately in the months after it has become as yellow as the others.

This is unfortunately inevitable as they embed some chemicals to reduce the yellowing (as in cars’ headlights) but after some time it “vanishes” (it consumes itself to protect the plastic from what I know).

So I would suggest you either go for a pricier clear case (like apple’s) or you can go for a coloured (maybe even personalised) case, as that won’t yellow. Another option would be to buy a cheap case you like as many times as you need to keep it always new and transparent.