This will be a long one, and not sure if the OP is still reading.
I own a GT430 Sport. Have driven an Evora GT410 (auto) before I bought mine. Have driven Cayman GT4 RS, not a vanilla GT4.
Also, other cars from this thread, 570S, A110.
The GT4 RS is an epic, epic thing. Would I buy it at double the price of my car? No. If it was the same price? Yes!
My biased opinion on the best back road car? Yep, my GT430 Sport.
How different is it compared to a GT410? It was a long time between driving the two, but my recollection says the GT410 is only a fraction off the GT430 experience, and the biggest difference IMHO is the Ohlins. Those things are a thing of witchcraft. Also, on some very bumpy Tasmanian roads, the GT430 handles bumps much better than the clutch of Porsche GTx cars I was travelling with.
I have it on reasonably good authority that any Evora is miles better in driver feel to the Cayman GTS. Again, no direct GT4 comparison. Opinions from friends who did drive my car and the 718 GT4 back to back was level at 1 - 1, that is, one loved mine, the other preferred the GT4.
I'd say choice between a GT410 and GT4 is going to come down to personal preference.
On the 570S - if I had the money, would love one, but feel it cannot be fully exploited on the road, so it would be either a waste or I'd lose my licence pretty quickly. At sane speeds the Evora is more engaging. I wouldn't sell the Evora to get one, but would love them side by side in my garage.
The A110 is a car I would buy and considered before buying the Evora. I had a Megane as a daily for some time. The A110 is pure joy on a bumpy back road. In Tassie, the A110 and Evora were the best cars to drive as they soaked up the bumps and were utterly engaging, at speeds that were not completely over the top.