The last time I checked, piped had a button right on the playlist page to export playlists as JSON. You can then switch instances and re-import that data back in
I’m not sure about subscriptions though, only really use piped for watching videos quickly or listening to music playlists
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