The 23XI Toyota is best on speed but the Penske you have has the best handling of all eight 2021 NASCAR's.
I'm probably going to purchase one NASCAR per update. Assuming we're in Update 10.1 on January 31st, 2022, that provides enough time to purchase one per update and get all eight in the garage.
Another strategy could be to wait for a sale on the NASCAR's, but that's likely to happen in the dying days and you likely won't have enough time to earn enough M$ at once.
But at the end of the day, unless you want as many cars as you can possibly get, want cars that will be forever gone after 1/31/2022, or love NASCAR's, there's no reason to get all eight, rather, like others said, focus on tuning or something else fun or profitable that is within the M$ realm.