Why yes Arcane Trickster is the way to go if you want a pure thief. Pure Swashbuckler might also be good.
For example on AT you can place your invisible Mage Hand next to any mob and always get Sneak Attack.
Assassin(3) is for Gloomstalker, Thief(3) is for all kinds of builds that want an extra bonus action. Both classes are very frontloaded, with little afterwards.
