Rules: You do Time Trials but you have a random number generator generating a number for every race (1-3). The number that shows up is the amount of mushrooms you will use for that Time Trial alone. You will keep doing this until you have used 50 mushrooms (or more).
Start Time: Start on the frame you select "OK" for the first Time Trial.
End Time: End on the frame you cross the finish line of the time trial of which you used the 50th mushroom.
More Information:
-The random number generator can generate a number between each Time Trial.
-You can only play a track once (OPTIMIZE THIS IF YOU WANT!!!)
-I don't know if it matters that the generators are pseudo random because there are only 3 numbers here.
-Random Number Generators:
https://www.calculator.net/random-number-generator.html
https://www.random.org/
http://www.randomnumbergenerator.com/

Also another idea is

Idle%

For this run you would do 48 tracks with both smart steering and auto acceleration on and you wouldn't move the left joystick at all. You can still rocket start, trick, and use items. The timing is exactly like a normal 48 track speedrun.

such a way to add trivial RNG to the one modality of the game without real RNG in a game with RNG in the main modality lol

i mean you would just reset until you get 3 mushrooms 16 times and 2 mushrooms once (or 3 mushrooms 17 times) but like the chances are low of that happening

And also I think Idle% is the better one of the two lol