I had a glass of Louis XIII Cognac a few years ago and was really surprised at how much better it was than a VSOP. As you say the tail lasted so long and was so rich and complex, I didn't expect it to be that incredible.
But on a more affordable level I recommend checking out https://www.cognac-expert.com. They do have the incredibly expensive stuff, but at a also a reasonable-price range that goes well beyond the standard Remy, Hennessy, etc. VSOP or even XO. A world to explore!