It's not gamebreaking. You still have all the normal restrictions rage gives you in order to get resistance to all damage. Also, an extra +1 to your Strength modifier isn't going to destroy the balance of the game.

Quote from Frozen_Fruit >>

After reading this, I am more confirm that Kalashtar Totem Warrior is a little broken. Using Crowd Control (CC) magic to knock Barbarian out of rage works on all Barbarian, not just Kalashtar Totem Warrior. However, Kalashtar Totem Warrior cab handles it better than other Barbarians. Many CC magic are Wis saving type and Kalashtar can use its reaction to have an advantage on saving throw while others can't. While int-save CC magic has the same chance to knock Kalashtar Barbarians out of rage as other Barbarians, it still takes less damage. When I say it is a little broken, I don't mean it is over-power. I mean it is a better choice than other Barbarians choices. When one choice is better than others, it promotes min/max and metagaming.

People say players shouldn't min/max or metagaming in D&D. Then why create one race that is much better than others and promoting min/max/metagaming?

Didn't see your reply there, sorry!

To clarify, I'm not arguing that the combo isn't strong, I'm just arguing that it's not as strong as it may seem at first glance, and that the racial changes in Tasha's don't make it much stronger in practice, which is what the OP was asking about. The combo is not dissimilar to the Bear Totem bonus itself which, while a great bonus, sounds stronger than it often is in practice (as it's no benefit at all if not raging, and while the extra resistances are great, a Barbarian's normal resistances are the ones they'll usually use the most often anyway). It's certainly not a weak combo either, but it has its limitations, and there are other ways to ruin that Barbarian's day; Banishment for example.

The most important things about playing a Barbarian in combat are either dealing damage yourself, or tanking damage, ideally in a way that prevents allies from taking damage instead. Bear Totem is already the tankiest personal Barbarian build, but it can actually be a double-edged sword in a group setting, as the less damage you take, the more likely an enemy may be to target someone else instead, adding more types of damage and flavours of spell that you can resist or save against means you might actually end up tanking less if the enemy can choose another target. Kalashtar doesn't make you especially more dangerous in terms of damage, and gives you no new ways to force an enemy to target you and only you. This is why it's often important to consider a build not just in terms of "raw" power as if every fight is one-on-one, but to consider what enemies (or your DM) might do differently in the group setting.

There's also an argument to be made that Kalashtar still aren't the best race for playing a Bear Totem Barbarian; a Half-Orc for example has Darkvision which Kalashtar don't (so can fight normally in poor light), have free proficiency in Intimidation (the go to non-combat ability of many a Barbarian, especially if your DM lets you use Strength as the base), have Relentless Endurance (basically a free Death Ward) and with Savage Attacks can be do even more damage with a Barbarian's already vicious critical hits. This is a really powerful mix of abilities ideal for a Barbarian, and already has the ability score bonuses you want without Tasha's Cauldron of Everything.

So if you want to know which race makes for the "most powerful" Bear Totem Barbarian, I don't think my answer would be Kalashtar and I wouldn't say it's broken (as in overwhelmingly good at something), though of course that's because my actual answer would always be "just pick whatever you want and have fun because there's normally no wrong choice in D&D if you're playing it properly". 😉