With a bit of work homeassistant can be a quite good voice assistant.
You can either revive some old android device and use that, or get an ECHO M5 for ~13€ and hook that one up.
You can even run some local Ollama AI and use that for the voice assistant nowadays. It's quite useful and home assistant can be integrated into music / audiobooks aswell with something like Music Assistant 2.0
Oh there are tools to download from spotify directly ;)
They pretend to be a player, download the songs into the offline cache and decrypt them using your login credentials.
I can DM them if you're interested. It's interesting because yt music premium still only givey you 256kbit/s but spotify premium goes up to 320kbit/s