If you're using it as backup for banking apps and the like I totally get not running a custom ROM on it! But you could also set that backup on something even cheaper, any one of the random not-bootloader-unlockable brands, and be left with the OP5 as a Linux phone. You're also right that the Linux support on OP5 is not up to standard yet, this is more of a question for the future if that support improves.