A lot of side quests in tier 5 (specifically Boss Race, Boss Elimination, Boss Slalom, and Boss Freestyle) require you to get 1st place to unlock all events, so you might as well try to get 1st in every event instead, which will also unlock you the Elite achievement.
Here are some tips and links to help you get there:
1. Make sure you have the difficulty set to "easy". This is found by going to Settings -> Difficulty -> Easy.
2. Make sure you learn the Boost Bonus, Boost Start, and Drafting skills when they are available. They are you highest skill priority and you should get them as soon as possible. They are available at levels 2, 5, 10, and 15. You'll need 3 SP (1 SP each) for level 2, 6 SP (2 SP each) for level 5, 12 SP (4 SP each) for level 10, and 18 SP ( 6 SP each) for level 15. I personally got Boost Start, then Drafting, then Boost Bonus each time I had the opportunity. Thank you nightw0lf for reminding me about this important detail.
3. Play it safe in freestyle events. Triple tricks are great when you can do them, but it's better to do a trick that you won't fail the landing on. Variety of tricks and consistency are more important than nailing a higher level trick.
4. With extra SP, prioritize level 2 tricks, then level 3 tricks, then level 1 tricks. You will be doing level 2 tricks more often than any other type of trick, and level 3 tricks fill the boost meter. NOTE: You will need to do a different trick off each jump or you won't get as much boost. Doing the same trick twice in a row, even in races, seems to give less boost than if you did 2 different tricks off each jump. Tricks that I used (you don't need all of these): Rock Star, Surf's Up, Lazyboy, Handstand, Hang On, Double Front Flip, Double Back Flip, Log Roll, Triple Front Flip, and Triple Back Flip.
5. Make sure you use a hydrojet that you are comfortable with. In my opinion, the Scorpion is the best hydrojet because it has the highest top speed, still handles relatively well, and has enough boost and acceleration to stay ahead. NOTE: The Scorpion is actually one of the cheapest hydrojets to max out, since you only need to purchase 8 upgrades (2 for each stat) as compared to most other hydrojets that require 40 upgrades (5 for each stat). The Cruiser is probably the 2nd best hydrojet in the game, but I felt it was too slow the few races that I used it. The Shockwave is probably the 3rd best, but I felt it was subpar by the time I got it (after beating the main storyline).
6. Make sure you know the shortcuts! In the last tier of side quests, it is pretty tough to win without knowing all of the shortcuts. While the easter eggs show you some of the shortcuts, it does not show you all of them, especially the ones that save you the most time. I saved a lot of time by using the shortcuts for The Ruins, Tropico, Downtown Rush, and Firewater. NOTE: The Unicorn Base shortcut for ignoring the jump to the left works only on the first lap. On the second lap, take the jump off the blue boat that crashes into the red boat.
Also, I found Alpha Station's shortcut too risky and I usually skipped it on my races. It is definitely not needed to get 1st in all events.
7. DON'T BOOST IN THE SLALOM EVENTS!
Don't miss any poles and don't boost and you will get 3 stars every time without any issues.
Here is a picture guide on the shortcuts: https://steamcommunity.com/sharedfiles/filedetails/?id=73410...
Here is a Youtube playlist on the shortcuts: https://www.youtube.com/playlist?list=PLoy5jXTGLDP36f7dTJPFW...
The Scorpion and the Shockwave only require 8 total upgrades to max out stats, while the Cruiser only needs 12 total upgrades to max out stats. However, I strongly suggest the Scorpion since top speed is a very important stat and can mean the difference between 1st and 2nd in a lot of these events.
EDIT 1: The links should be working properly now.
EDIT 2: Added in more sections: SP priority, freestyle advice, trick advice, and slalom advice.