Concussive Shots is meh, you may consider dropping it. But Elemental Bolts is a true powerhouse, so take it as easrly as you can - 14 or 16 lvl. Sharpshhoter is useful too, since you are going to need additional crit damage.
My last crossbow build was something like that
http://underrail.info.tm/build/?GQMHBwMQAwcAAMKHAAAAwodfRgAybnhVUHgAAAAAAAAoMQEiNTgzS2owHScmXQAs you noticed, i don't have Deadly Snares, mostly becayse it's usefullness is very limited. First, because enemy that is caught in a trap is not a danger to you for the most time and you can wait before finishing him. Second, because my build had critical chance around 75%, so most of my shots were already critical. For the same reason you probably should avoid taking Ambush, because you crit chance will already be close to 100%.
I used two crossbows: Pneumatic Cyclon + high res scope and Monsoon + superstring + high res scope. Other gear was infused rathound leather armor and seeker goggles, all increasing crit chance. With all of that it is possible to have a very high crit chance and with Elemental Bolts it will even negate most glaring crossbow weakness - high mech resist. With Expedition, you can boost you crit even higher, to almost 100%.
Crossbow is the strongest when you start the combat out of stealth (though it will be a bit boring after a while, since you are making mostly dame thing over and over). For other battles there is food that boosts your init (which is already decent). Maybe even take Trigger Happy in place of veteran feat to be sure.
Shock bolts are best out of bunch. They have nice damage and they stun both oraganic targets and robots. Couldn't be better, really. And you can even buy them from vendors.
As far as crafting goes, you should get a very good shield, you will need it for a couple of tough fights. You should have enough chemistry to craft mkIII fire bolts (60 effective chemistry i think), they gonna serve you great in a final fight. Corrosive Acid is also good, since it reduces the resistances, takes a bit more chemistry (around 70-80). Poisons...are meh imo, at least compared to elemental ones. The only solid use i had is hunting bladelings with them (and gas grenades for a big fight).
P.S. Ambush became better after Styg tweaked illumination mechanics.