The whole concept of backstab and sneak attack doesn't make sense. They really should be either synonymous with each other or at switched in usage. A sneak attack, by direct definition of the word, is just an attack done in a sneaky way. It doesn't neccessitate extra damage, just an attack done sneakily. Hence, it's just an attack with higher likelihood of connecting. A sneak tackle is a sneak attack and is more likely to hit, but not more likely to deal damage.
I know this is all covered easily alreay by giving all attacks, while hidden, advantage. So why not just replace/rename sneak attack as backstab. Then to fix the loss of sneak attack as a skill, give all rogues an ability to gain advantage on all distracted opponents (new sneak attack). Distracted opponents include opponents engaged in fighting other targets, or hostile opponents that targeted things that didn't target the rogue last turn. This should replace the Lv 1 Attack Bonus that all rogues get.
Backstab should then replace the current sneak attack. It would be the same exact thing as sneak attack, but renamed. However, I also think that the damage is quite a bit too much. IMO it should be dropped one die type per roll. ie in lvl 1, it should be 1d4 instead of 1d6.
This helps balance it on assassins which can now have 'advanced backstab' as a skill which just upgrades the dice one type higher (back to original sneak attack levels). I just don't see why Assassins, who specialize in killing one target at a time don't have increased damage compared to base thieves. It's not very 'assassiny'
Or maybe you can at least add poison mechanic for assassins as a free skill. They spend money to make poisons that deal 1d4 dmg per turn unless the target makes a fortitude save. This poison costs 10gp of materials to make and can be upgraded to the same number of dice they use to backstab by spending an extra 10gp. IE by making a poison costing 30gp, the poison does 3d4 points of damage per turn.
So the changes in short are:
1) Sneak Attack renamed to backstab
2) Backstab is now reduced by 1 die type. From d6 to d4 on first level, etc
3) New 'Sneak Attack' is now a class benefit replacing the Lv 1 attack bonus that all rogues get
(Optional to 'fix' assassins):
1) Assassins can make poisons that deal 1d4 per 10gp spent on manufacture
2) Assassins gain a new skill, 'advanced backstab'. This increases 'backstab' (formerly sneak attack) by one die type (as it is in current version of sneak attack)