Thermometer is the most important accessory for BBQ, especially with the smokin-it cookers. To steal a phrase from others on here - if you are lookin you ain’t cookin!
So, I Have a Thermoworks Chef Alarm,
https://www.thermoworks.com/ChefAlarm That works great. There are others out there that offer more probe connections that I have interest in (FireBoard) but just don’t need right now. And yes, drop the probe wire down the vent hole and into whatever you are cooking, brisket, pork, chicken, turkey. And then let it go to the desired internal temp. Saves you from opening and losing the heat.
Where to probe is based on what you are cooking, but poultry I usually probe the dark meat, so in the thigh. Brisket I run two probes, sometimes three, one for the flat and one in the point, and the pork butts, just anywhere as long as it’s in the middle.
As for box temp - I don’t worry about watching that. I do have a digital model, so I see what my temp is, but for analogs, the experience smokers on the board say don’t watch it. It’s pointless and can cause more concern than anything. But they can provide that feedback as well.
Hope that helps. I can’t help on the jerky - although I did make my own fan out of a computer fan and piping, but I use that for lower temp smokes on bacon and sausage.