The problem with a STR based build is that Ratfolk have a racial penalty to STR, and the damage is halved for the offhand weapon. Double slice will take care of the STR bonus for the offhand weapon but the bonus from power attack would still be less.
With Unchained Rage the bonus is not cut, nor would the bonus for Powerful Stance. Both of those give a scaling bonus that goes up as the character levels. Both Favored Enemy and Sneak attack are situational bonuses that will often not apply. While unchained rage is a limited resource it can be used anytime the character is in melee combat or with thrown weapons.
All very fair points! And why I prefaced the suggestions as flavor-oriented. Overcoming a racial penalty isn't all that uncommon when ignoring min-max builds. Favored enemy is again flavored for a character known for grudges, and the sneak attack is much more reliable for a ratfolk (pseudo-skaven) using scurrying swarmer, assuming there is another melee character in the party. Slayer talents help build TWF feats faster, as barbarian rage powers just don't help that build much.
But I will add, Unchained barbarian is good for another reason: much simpler than the original. OP notes that they are new to PF, so whether as straight class or dip, unchained is easier to math. And beast totem chain into pounce is always lovely.
I appreciate the feedback and suggestions! I'll make sure to try out all of the suggestions (which were much appreciated) and see what works best. Until then, I will still look at and take any advice people offer.