You don't put your seed into a Trezor server if you need to restore it... and your recovery seed uses the "standard" BIP39/BIP44 configuration... so if Trezor goes out of business or your lose it, you can restore to a compatible wallet or another hardware wallet...
Have a read of these two:
https://doc.satoshilabs.com/trezor-user/emergency.html and
https://doc.satoshilabs.com/trezor-apps/index.html#recovering-funds-without-trezor-deviceThe whole idea of a hardware wallet, is that the seed should only exist within the wallet itself, or written on your recovery sheet... it should never be stored digitally. So aside, from the recovery sheet, the device is "truly independent of outside factors" as you put it.