Besides, why not just extend your seed with a complex passphrase and not use the SD cards at all?
I don't think this will solve the problem, the philosophy of SD card is to protect against
physical attacks of the device.
If someone with the equipment and expertise required was able to physically attack your Trezor, then they are "only" able to extract your seed phrase and access your base wallet, or unlock with your PIN and access your base wallet. Any passphrase protected wallets would remain both hidden and protected by the passphrase (which should obviously be long and complex enough to be resistant to brute force attacks).
For example, if someone managed to find your device and knew PIN code, then you lost your money, here comes SD card role as a second password.
Again, only your base wallet, not any passphrased wallets.
But, you can quite happily use both the SD card and one or more passphrases, so no need to choose one or the other.