As for the firmware update, I have two options, one through SD card and the second through USB connection, which one do you think is better and safer upgrade way?
Is there any Keystone user on here? What's your user experience so far? Mine is Keystone 3 pro
I don’t know much about this hardware wallet but here is a thread by dkbit98 on this particular
Keystone 3 pro hardware wallet you can simply ask your question there.
In the upgrade of firmware I think both the usb and SD card are same, if you’re insinuating on catching any malware, make to use an old offline device at least. But AFAIK the private key which is the most important thing stored on the hardware wallet is not transferred when the card or USB is plugged in.
Another advice is, do not rely totally on this hardware wallet device, make sure to back up your seed phrase in case there is loss or damage to the device most especially since it is a touch screen sensitive a fall could cause its damage, but with your seed phrase backed up properly (best offline) you can recover the wallet on another device.