I do a lot of varnish on my boats. The epiphanes is a relatively thick varnish and it is very slow drying. Slow drying will result in more dust collection. I have to varnish outside on my larger boats. If using the Epiphanes, I have to have the longer days of the spring. I have tried to varnish in the fall and the days are too short, it will blush. I think that you will find that 400 paper is bit fine for varnish work. The number of coats that you are doing is about right. On new or stripped wood I do 10 coats. Filter your varnish before using. I have tried a badger hair brush but went back to foam.
You might consider a faster drying varnish for a topcoat. Varnish will drive you crazy sometimes!
You definitely do not want to leave the truck outside regularly. I say that horizontal+varnish+ teak+ sun is mission impossible!