The way I have approached these grinds in the past Forza games has been to start from the lowest car class and work my way up from there. So, after completing the initial "introduction" of the career mode, I decided to create a list that breaks down each Volume and Showcase into tiers based on the car classes available in each. It might not be the most efficient approach, but I like it as I feel like I'm progressing from the slowest cars to the fastest without having to backtrack much to use slower cars. This is a grind regardless.
Anyway, without further ado, here is the list! I hope anyone who decides to use this approach finds it useful. I would recommend copying it into a word processor or something and marking off events as you go along.Volumes