It did take them a year to "get fuel tanks right". That doesn't mean they didn't know how to make a fuel tank. They were optimizing the design as much as possible trying to go for the best mass fraction they could while still having the necessary strength. They had some pretty requirements in that regard because of their performance goals. All of those burst tanks were steps to the proper design that could meet their goals at a given weight.

I think you underestimate how difficult those were. If it were easy to make a low mass pressure vessel that can handle cryogenic temperatures, and do so at a mass fraction lower than existing designs while also being reusable, then they wouldn't have had to iterate so much. Such a design would have already been in use if it were that easy. Which in a circular way means they probably would have been doing all of this anyways to hit even higher performance goals.