No, it should not be a toggle. It should be automatic. The first attack which hits. If your main hand misses, but your off-hand attack hits, the off-hand gets sneak attack.
A toggle is an extra thing to manage, and I don't think that having to manage that yourself is something you should need to do, when the game could easily handle that for you.
But but but If don't want to manage the toggle Just... Leave It on?
I want to manage because that is something Very importante for some rogue builds to work and is How the sneak attack mechanic works by the PHB, why change tô something more limiting. A toggle gives everything you are asking for, by just leaving It on, but also makes sneak attack work according to rules, so why not toggle?
Again the same example, If there is a low HP enemy and I attack I don't want to apply sneak, so I disable the toggle, otherwise you waste your damage. And having the game choose for you is also bad, because sometimes you don't aim to kill, you aim to get unconcious, and sneak could ruin that, again a toggle would fix it, where automatic only adds different problems
