I do recommend encrypting your wallet, but I have another guide to cover that.
I'd be careful leaving any trace of your passphrase on the Raspberry Pi. Instead, it's better to have a watchdog process to inform you if the qtumd process goes down or if the qtumd PID changes (so that you can manually bring it up again, then securely unlock it). This topic is a bit more advanced though... but you have the right idea.