If MIT AppInventor is still kicking around, you should be able to use it for this… although sadly you won’t have access to the source code since it’s a Scratch-like way to create apps.
By default the Android voice assistant uses Google tech AFAIK, if you’re after a truly source-available solution then there’s ”Futo voice input" to handle STT, and “RHVoice” to handle TTS - though these would still need a HTTP API bridge to do what you want
Flash drive hidden under the carpet and connected via a USB extension, holding the decryption keys - threat model is a robber making off with the hard drives and gear, where the data just needs to be useless or inaccessible to others.
There’s a script in the initramfs which looks for the flash drive, and passes the decryption key on it to cryptsetup, which then kicks off the rest of the boot mounting the filesystems underneath the luks
I could technically remove the flash drive after boot as the system is on a UPS, but I like the ability to reboot remotely without too much hassle.
What I’d like to do in future would be to implement something more robust with a hardware device requiring 2FA. I’m not familiar with low level hardware security at all though, so the current setup will do fine for the time being!