If you can't decide what class you want to focus on and you like them all, then just play them all.
In this game you get bonuses for leveling up classes. For every 5 class levels you get a slight boost in some stats so in the end you will need to level them up anyway.
You shouldn't get that tired of switching classes really because you will be switching classes constantly forever to suit the needs of the different things you do. Things like Class Loot System also make it important that you can switch between various classes and still be helpful.
Also lots of gear are useful cross classes. With Magic DPS, it seems like you have 4 choices but 3 of those classes (Magician, Illusionist, Warlock) could use the exact same equips and be super useful.