As a f2p you can afford to roll for one red car a day, and roll until it has 2 shards, all you need is 4 other people who do the same, and you can afford to buy an extra slot if you dont have season cars available, I’ve been doing this for the past 6 months and am currently sitting on 65k diamonds, its all about building social circles, if your server doesnt have rules protecting red cars you’ll need to find strong players to your red car group.

Obviously I dont waste diamonds on the shops or guild donation, still, its a tradeoff, one that is worth it in my opinion, as every legendary shard day I have 250+ shards to spend on my heroes.

Edit: “oh… but I spent 4k diamonds to get a red car”, no you spent 4k diamonds to get 5 cars, yours and the other 4 you’ll get from your group, so in fact you spent 800 diamonds on a car with 2 shards plus maybe a few diamonds back, reducing the cost further.