5e D&D classes have major class features (post archetype selection which varies from level 1 to 3) at level 6-7 and 10 in the level range Larian wants to implement. Saying that once you pick an archetype, that's pretty much your class from now on, you just get more of what you already have.
Also, the XP table isn't flat, level 1 to 3 are pretty fast, then it start to slow down and I don't think the plan is to reach the level cap at the last battle of the game.
