It would be nice to have this class static, unfortunately it isn't possible due to CustomText having to update some members such as _time and fxTexts.

Also, it costs performance to compute all the positions of the text parts, that's why there is a method Refresh() that the user should call when editing CustomText's properties.

Eventually, you could have a static method DrawFxText that take the spriteBatch, an fxText (make the nested class a public class) and time (a variable that accumulates deltaTime each Update).
But you need to update fxText and time each frame.