so how does a grid autosport speedrun work?

Hi, depends on the category:

• Career: Finish the GRID Legends Cup
• All Disciplines: Place first in every single cup
• Single Disciplines: Place first in all cups of one category, such as Touring, Endurance, etc.

Speaking about strats, it also depends on the category:

• Career: afaik you advance by placing at least third
• Pretty much every other category is first place only

You can skip races so save time, so it's best to check which races take the longest and how high your chance is of completing a cup in 1st or 3rd place by skipping X races. That's pretty much the main strategy aside from driving fast.