An at least pretty good solution to computing parallel curves of Béziers is given in [1]. This can give very accurate results with a modest number of segments, using techniques that are fast to evaluate and not terribly complicated. I know it's possible to do better in terms of number of segments, but it's not obvious to me how to make that faster than doing curve fitting and error bounding.

[1]: https://raphlinus.github.io/curves/2021/02/19/parallel-curve...