At this point (danbruc correctly covered the first part of my point) you have basically reinvented an arbitrary precision library by storing the numerator, deno

At this point (danbruc correctly covered the first part of my point) you have basically reinvented an arbitrary precision library by storing the numerator, denominator and then having a utility function to choose the exact behavior of the result when converting back to numbers you use for storage and show to the user. This isn't a fixed point number system, because you are now using a non-fixed point number representation (e.g. two numbers; which can't be represented in fix point because of their repeating nature) to represent your fraction. Trying to get all of that behavior right is more error prone than just using an arbitrary precision library in the first place.