What if my NAS controller fails

8 years ago I got a NAS QNAP TS420 and filled it with 4x 4TB drives and configured it in raid 6. I am extremely happy with this device and it has given me no trouble and if a drive fails I’m quite sure it will survive after replacing a drive (not happened up to now) but as this device is getting older what would happen if the NAS itself failed? I wouldn’t be able to transfer my raided drives to another device due to raid implementation differences so my question is what is the best way to increase my failure resistance?