Just returned the car. Nothing was damaged and I returned it with same gas amount as received. No issues or additional charges. There was a large hold on the card for not paying additional insurance, but I didn’t mind it. If booked with Sixt website, required third party insurance is included. Since I was skeptical, I did take photos of all documents, and photos and videos of the car in case I was charged wrongfully.
Car conditions - it did have the engine light on the whole time, and it had 113k miles, but it worked well driving to La Paz and Cabo Pulmo.
The only thing I wasn’t pleased about is the various airport desk folks trying to get you to spend money on their “deals”. You’ll see them after the location where you submit your customs form. Just ask them for the car rental company pickup location and leave. Once you get there, you’ll see they have a car with a small Sixt sticker and the driver will be holding a laminated Sixt logo. It’s like a 5 minute drive to the car rental spots, free of charge.