Settings for damage given and taken and for HP are already existing for players, NPCs and monsters but not for pets and Thralls. And there is a Setting for friendly fire.
99% of balancing problems usually occur when specific weapon A is doing too much damage, while specific weapon B is underperforming. Since you are proposing to adjust damage by source type, I can’t see how this slider will help with balancing.
This is true - but the thrall with his heavy armor and tons of hitpoints still acts as a tank while you can do the damage to the enemy. Even at 50 % NPC damage, thralls are reasonably effective at hitting, and no boss I’ve fought (I haven’t tried the two new dungeons yet) gets even close to killing the thrall unless I try to be helpful and try to apply Sunder to the boss and end up cracking my thralls armor because the two-handed hammer seems to hit an unintuitively large area.