You are talking about removing Dex bonus to AC, which every class gets, to make Barbarian able to completely dump 4 ability scores and not even need items (at least a fighter needs nearly 1k gold to get into heavy armor that will compete with this barbarian completely naked). Barbarian is not broken in a bad way currently compared to any other martial. A monk might keep up with AC, and maybe even damage output (but I’d argue being locked into monk weapons means they won’t) but they’ll have at best 60% of the HP a barbarian has (who is also taking half damage from non-magical sources a majority of the time) and still have to put points into 3 stats to stay relevant.

Also, BG3 is not a great source for comparison… I have a bard at level 5 with a 21 AC. The more “tanky” classes I have played were all around 23 AC at level 7. There are a lot of magic items in the game that stack AC and you are absolutely swimming in them by level 8. Every party I’ve played through by the end game it was 4 characters with 23-26 AC across the board. My wizard was 24 AC (25 if standing in low light) by level 10…