Assassin is an Expert class. Although your route looks more like it should be Page > Thief or Myrmidon > Assassin > Trickster.

Thief and Myrmidon are both Basic level classes, Assassin is Expert, and Trickster is a Master class.

I'd also like to take the chance to note that you can deviate from this as long as you obtain the Talents to do so. For example you have Sword and Axe as a Mercenary, but also pick up Riding talent somewhere along the way, Cavalier or Paladin becomes open to you as well.

Obtaining your first class of any tier is free, but reclassing is not. However, you get to keep the Abilities you learn from all the classes you have used before, so if there's a class ability you really want or need, it's not a bad idea to reclass for it.