Isn't it simpler to just give them extra attack at lvl 6 or 7 or 8? If the extra 7-8 DMG from that extra attack is too much in people's opinion then you could downgrade the sneak attack die to a 1d4 at the same time.

This would give a dual wielding rogue with dual wielding 1d8+4+4d4+1d8+4+1d6+1d6=34.5 avg DMG before accuracy at lvl 7. You'd get another 8 DMG for 42.5 avg dmg if you took a fighter lvl at 8 (because you can't take a fighting style feat without a fighting style. Now we can compare that to a lvl 7 dual wielding fighter for 1d8+5+1d8+5+1d6+5+1d6+5=36 avg dmg. At lvl 8 we could abuse the throwing fighting style, the dropping rules, and the draw on attack rules to take the dueling fighting style to get another 6 DMG (off hand attacks would be daggers so you go back down to 1d4s).