as a former downhill mountain biker i would possibly suggest that a lot of the time longer isn't better lens wise for mountain biking in this country (unless you go somewhere really open like fort bill), most of the time you will be pretty close to the action dodging between trees in heavily wooded areas so fast zoom lenses with constant apertures are a good way to go, the nikon 17-55 f2.8 being a good example, this is a lens that would give an obvious upgrade on you current kit lens that you will quite possibly consider down the line anyway and it can be bought relatively cheaply second hand. Panning is a popular technique for mtb shots so look into some tutorials on that, getting some motion in the shot is always a good way to go and also try and leave space before or after the rider so it looks like they are riding into the shot or where they have come from (if its an interesting bit of trail that is).
FWIW when i used to go downhilling and take my camera with me is was a nikon d90 which is a lot less capable in low light than your d3400, i also only had the kit lens at that stage and made a boat load of mistakes when trying to get decent photos with far more misses than hits. What i would say though is that it is perfectly manageable to get decent shots out of even a modest camera body and lens but you do have to put some work into learning a few techniques to help you. Figuring out good positions on the trail helps too, and as previously mentioned look to get low down or high up if you are happy to climb trees. Standing head on makes for very dull shots.
A couple of other things, positioning yourself low down on the inside of a bend/berm can make for a good shot, if your other half is comfortable getting air and whipping the back end out/doing something other than a dead sailor in mid air then that works too. Also remember that things like rock gardens in trails will usually slow all but the fastest riders down so if you are struggling to get to grips with riders going at full pelt then they can make for a good place to try and get some slower speed shots.
I have tried to keep these examples generic to most mountain biking places as you didn't specify what discipline your other half rides but most trail centres these days have jumps and drops albeit to a smaller scale than downhill courses do.
One last thing, if there is places where it looks like people might crash make sure your camera is always ready to capture the special moment when you can see their expression change from "this is awesome" to "oh f**k"