All the magic item tables have "something" strength enhancing on them. The very rare variants of the belt are competitive with the level 20 barbarian talent, and don't cost you feats. Getting strength instead of dex directly costs you AC, and in the early game, you probably need that AC, furthermore, with Dex you can use a bow.

Potions of Giant Strength start at uncommon, Gauntlets of Ogre Power are uncommon (str 19), the belts are scattered across Rare (21) - Legendary (29), and werebears are CR 5 (chaotic good, 19 strength when in werebear form).

Your GM would likely need to fudge rolls to prevent the party from finding "something" strength enabling over the course of your first 5-8 levels (Rage goes from +2 to +3 damage at level 9). If he feels like intentionally screwing with you, it will become obvious when the party gets its' first rare or two that nobody can attune to, and the GM refuses to allow the party to quest to trade it for something that would really benefit a character, even something of a lesser rarity.

Guidance isn't something you would use in combat, but a quick prayer to your god is fine. "I ask my totem spirit for guidance and attempt to bend the bars" so you can throw a d20 and a d4 instead of just a d20 is good roleplaying. Given that the barbarian is diving into close combat, if you pray "bear spirit guide my hands" as you kick the door in, a the DM says roll for initiative, that's probably going to be where you use it, and you won't waste your action on a cantrip in combat unless there is literally nothing else for you to do.

Thorn whip being off of wisdom mod + proficiency bonus makes it less damaging than a ranged attack means that it will almost never been your first-line attack, which is fine. If you are using it though, it's because you are not going to rush whatever you're casting on, the target is unusually vulnerable (blindness, paralyze, faerie fire from the bard or druid, etc), or it is simply out of reach and you're out of arrows and javelins.

There was someone who did the math of dex vs str on the wizards forums in one of the barbarian guides and basically found that the AC bonus and +to hit of going for dex instead of strength basically comes out even. Also, I don't know about you, but I'm more likely to defensively cast rage for the damage resistance than because I'm depending on an extra two points of damage this round.