I agree that having it be automated is the way to go - keep it simple. The trick is to make it an automatic effect that triggers on a hit, not an action which is separate from a weapon attack and thus can be exhausted. "The first time you hit with a weapon attack (and the other conditions are met) each turn, deal Xd6 additional damage." That's a little less flexible than in 5e, but ease of use outweighs that tradeoff.
The most important thing is that I don't think we need a separate button for sneak attack, just like we don't need different buttons for "attack" and "attack with bonus damage from Hex" for warlocks.
