A month ago, I rented an SUV from Australia too.. in Sydney.. an Isuzu Mu-X.

If you told Chase there was pre-existing condition and sent them the photos, they would deny Sixt's ability to charge your card and more likely refund the $3k if you can show it was for the claimed damage.

I have never been in that situation though.. but that's what I would have done. Best of luck.