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*** Spoiler - click to reveal ***E Class
[] Volume 1: Super Street - Classic Compact
D Class
[] Volume 1: Super Street - Compact Sport Coupe
[] Volume 1: Super Street - Hot Hatchbacks
[] Volume 2: Sport Icons - Historic American Muscle
[] Volume 3: Grand Touring - Vintage Touring
C Class
[] Volume 1: Super Street - Japanese Street Kings
[] Volume 1: Super Street - Rally Day Heroes
[] Volume 2: Sport Icons - Sport Utility Juggernaut
B Class
[] Volume 1: Super Street - Ultimate Spocom
[] Volume 2: Sport Icons - Modern Sport Legends
A Class
[] Volume 2: Sport Icons - Ultimate Sport Machines
[] Volume 2: Sport Icons - Executive Heavyweights
[] Volume 2: Sport Icons - Track Toys
[] Volume 4: Professional Racing - Historic Sportscar Racing
S Class
[] Volume 3: Grand Touring - Sport GT
[] Volume 3: Grand Touring - Exotic GT
[] Volume 3: Grand Touring - Birth of the Supercar
[] Volume 3: Grand Touring - Ultimate Grand Touring
[] Volume 4: Professional Racing - V8 Supercars
[] Volume 4: Professional Racing - Global Touring Cars
[] Volume 4: Professional Racing - Formula E
[] Volume 5: Ultimate Motorsport - Legends of Grand Prix
R Class
[] Volume 3: Grand Touring - Modern Hypercar
[] Volume 4: Professional Motorsport - Modern GT Racing
[] Volume 4: Professional Motorsport - Early GT Racing
[] Volume 5: Ultimate Motorsport - 1970s Grand Prix
[] Volume 5: Ultimate Motorsport - Can-Am
P Class
[] Volume 5: Ultimate Motorsport - Endurance Prototypes
X Class
[] Volume 5: Ultimate Motorsport - Modern Grand Prix
[] Volume 5: Ultimate Motorsport - IndyCar
Showcases*** Spoiler - click to reveal ***E Class
[] Top Gear Showcase - Underdog Challenge - Hot Rods
[] Top Gear Showcase - Underdog Challenge - Micro Cars
[] Top Gear Showcase - Underdog Challenge - Trucks
[] Top Gear Showcase - Ford Passing Challenge
D Class
[] Bondurant Autocross - Mazda MX-5
[] Factory-Spec Racing - Mazda MX-5 Spec Cup
[] Moments in Motorsport - America’s Muscle Car Battle
C Class
[] Bondurant Autocross - VW Golf R
[] Top Gear Showcase - Land Rover Passing Challenge
[] Top Gear Showcase - Jeep Passing Challenge
B Class
[] Track Day Shootout - Ultimate Spocom Shootout (C Class, except Nissan 370Z is B Class)
[] Moments in Motorsport - Classic Road-Racing Sports Cars (D, C and B)
[] Top Gear Stig 1-1 - Audi RS6
[] Top Gear Stig 1-1 - Alfa Romeo 4C
[] Top Gear Stig 1-1 - BMW M4 (Complete High Speed Chase - BMW M3 Sprint to unlock)
[] Top Gear Showcase - BMW M4 Bowling (B Class)
A Class
[] Moments in Motorsport - The Birth of Grand Prix Racing (A or B)
[] Track Day Shootout - Sport Icon Shootout (B: Audi RS 4 Avant, BMW M4. A: Jaguar F, Mercedes AMG GT)
[] Track Day Shootout - Modern American Muscle Shootout (B: Ford Mustang ’15. A: Chevy Camero SS, Dodge Challenger ’15)
[] Bondurant Autocross - Mercedes AMG GTS
[] Bondurant Autocross - BAC Mono (Complete Top Gear Showcase - Track Day Masters using BAC Mono to unlock)
[] Factory-Spec Racing - Lotus 2-Eleven
[] Factory-Spec Racing - Caterham R500
[] Moments in Motorsport - Ford & Ferrari at Le Mans
[] Top Gear Stig 1-1 - TVR Sagaris
[] Top Gear Stig 1-1 - Jaguar F-Type (Complete Top Gear Showcase - Jaguar F-Type Bowling to unlock)
[] Top Gear Stig 1-1 - Ferrari FF
[] Track Day Shootout - Track Toys Shootout
[] Top Gear Showcase - Mercedes E63 Bowling
[] Top Gear Showcase - Jaguar F-Type Bowling
[] Top Gear Showcase - Track Day Masters (Use BAC Mono to unlock Bondurant Autocross - BAC Mono)
S Class
[] Top Gear Showcase - Power Lap Masters (Jaguar/Mercedes are A. Others are S)
[] Bondurant Autocross - Chevy Corvette Z06
[] Endurance Racing - Bathurst 100
[] Factory-Spec Racing - Formula Ford
[] High-Speed Chase - Ford GT Sprint
[] High-Speed Chase - Lamborghini V12 Sprint (Complete Top Gear Stig 1-1 - Lamborghini Huracan Stig 1-1 to unlock)
[] Moments in Motorsport - 1960s Grand Prix Legends
[] Passing Challenge - 2017 Ford GT
[] Top Gear Stig 1-1 - Corvette Z06
[] Top Gear Stig 1-1 - Lamborghini Huracan
[] The Race Driver Experience - Global Touring Cars Nurburgring Invitational
[] The Race Driver Experience - Global Touring Cars Brand Hatch Rain Invitational
[] The Race Driver Experience - V8 Supercar Bathurst Invitational
[] The Race Driver Experience - Formula E Long Beach Invitational
[] Track Day Shootout - Sport GT Shootout (S, except Aston Martin Vantage S is A Class)
[] Track Day Shootout - Exotic GT Shootout (Complete High Speed Chase - Ford GT Sprint to unlock)
R Class
[] Moments in Motorsport - Unrestricted Early GT Racing (S and R)
[] Endurance Racing - Nurburgring 125
[] Endurance Racing - Catalunya 100
[] Endurance Racing - Road Atlanta 100
[] Endurance Racing - Spa 100 (Complete The Race Driver Experience - Modern GT Spa Rain Invitational to unlock)
[] Factory-Spec Racing - Ariel Atom (Complete Top Gear Stig 1-1 - Ariel Atom Stig 1-1 to unlock)
[] Factory-Spec Racing - Radical SR8 RX
[] Factory-Spec Racing - Ultima GTR
[] High-Speed Chase - Chevy Corvette Sprint
[] High-Speed Chase - BMW M3 Sprint
[] High-Speed Chase - Ferrari V12 Sprint (Complete Passing Challenge - Ferrari LaFerrari Challenge to unlock)
[] Moments in Motorsport - Grand Prix Heroes of the 1970s
[] Moments in Motorsport - Vintage Experimental Prototypes
[] Passing Challenge - Ferrari LaFerrari
[] Passing Challenge - Ultima GTR
[] Passing Challenge - Radical SR8 RX
[] Passing Challenge - Hennessey Venom (Complete Passing Challenge - Bugatti Veyron to unlock)
[] Passing Challenge - McLaren P1
[] Passing Challenge - Koenigsegg Agera (Complete Passing Challenge - McLaren P1 to unlock)
[] Passing Challenge - Bugatti Veyron
[] Passing Challenge - Pagani Zonda (Complete Passing Challenge - Koenigsegg Agera to unlock)
[] Top Gear Stig 1-1 - Koenigsegg Agera
[] Top Gear Stig 1-1 - Ariel Atom
[] The Race Driver Experience - Modern GT Spa Rain Invitational
[] The Race Driver Experience - Modern GT Daytona Late Night Invitational
[] The Race Driver Experience - Modern GT Sebring Late Night Invitational
[] The Race Driver Experience - Modern GT Nordschleife Late Night Invitational
[] Track Day Shootout - Hypercar Shootout
P Class
[] Endurance Racing - Sebring 300
[] Endurance Racing - Le Mans 250
[] Endurance Racing - Daytona 300 (Complete High-Speed Chase - Chevy Corvette Sprint to unlock)
[] Moments in Motorsport - Early Endurance Prototypes
[] The Race Driver Experience - Endurance Prototypes Spa Late Night Invitational
[] The Race Driver Experience - Endurance Prototypes Le Mans Late Night Invitational (Complete Moments in Motorsport - Early Endurance Prototypes to unlock)
[] The Race Driver Experience - Endurance Prototypes Sebring Rain Invitational
X Class
[] Endurance Racing - Indianapolis 250
[] Factory-Spec Racing - Lotus E23
[] Passing Challenge - Lotus E23
[] The Race Driver Experience - IndyCar Indianapolis Invitational
[] The Race Driver Experience - Modern Grand Prix Yas Marina Invitational (Complete Moments in Motorsport - The Birth of Grand Prix Racing to unlock)