Of the "big three" saves, CON and WIS saves are important at the higher levels, and CON is even more important if you have spells with concentration like Hunter's Mark. That said, DEX is both integral to what you do and such a great/overloaded ability that I would not delay it at all. Don't worry about it - you're playing a ranged character and it's not the end of the world if you occasionally lose your hunter's mark. I'd go up to max DEX, and if you're looking for other things look at Sharpshooter. (The other feat oft-suggested for ranged, Crossbow Expert, would not be a good fit because you are already competing for your bonus action.)
As a side note, you mention 3d6 sneak attack and "able to use of most attacks". In 5e, you get that at most once per turn, not on every attack that hits. Nothing wrong with multiple attacks and SA - good way to make sure it gets delivered. But it's not adding to every attack you do which may cut down on it's utility some if you were expecting it to like in earlier editions.
