I'm broadly unsurprised: they're not experts in GT by any stretch. We are, but frankly I don't even know what the problem is, so PS support has no hope.
There's no such thing as a "best cars file" and indeed your GT7 save game isn't even stored on your console at all. Your GT7 game save data - with your progress, credit balance, garage, results, and so on - is stored remotely, on Polyphony Digital's servers, which prevents both cheating and file corruption. It's checked when starting the game up.
Could you better describe exactly what the issue is? By that I mean explain what it is that you are doing and what it is you see as a result of doing it that leads you to believe your "best cars file gets corrupted"? There's only one thing I can think of, but it wouldn't lose you any credits or cars, so I need more information.
You can use your PS4/PS5 console's inbuilt capture function (called Create on PS4 or Share on PS5) to capture screenshots which you can transfer off your console with a USB flash drive or to your PS App and upload here - and screenshots are much better than photographs taken of a TV/monitor screen as they are a standard resolution, not subject to reflections, and don't crop off important parts of the screen.