The one solution I can think of which is not involving adding any hardware is using Spotcast to start Spotify playback on your tablet.
Haven't tried exactly this setup, but based on their description it should work.
Home Assistant is open source home automation that puts local control and privacy first. Powered by a worldwide community of tinkerers and DIY enthusiasts. Perfect to run on a Raspberry Pi or a local server. Available for free at home-assistant.io
The one solution I can think of which is not involving adding any hardware is using Spotcast to start Spotify playback on your tablet.
Haven't tried exactly this setup, but based on their description it should work.
Either you have a smart speaker that is controlled by HA, or you have dumb speakers connected to something smart that is controlled by HA.
Ordinary Bluetooth speakers are dumb so they need to be left on and connected. Can your tablet run some kind of audio server in the background?
Look into a raspberry pi with Volumio. You can play music through the headphone jack that way (assuming you get a Pi with a headphone jack).
Home Assistant has a Volumio integration but I'm not sure what it's capable of. You can also run Music Assistant (available as a HA add-on), with which you can set up Volumio as a DLNA player. You'll have to try it out and see if volume control is available, but I can tell you that using Music Assistant with Chromecast lets me control volume through Home Assistant.
Edit: maybe a pipe dream, but maybe Music Assistant can control the Spotify app on your tablet? Only one way to find out.
You can also use Plex Meets Homeassistant if you make the pi a plex player.
+1 for this.
I have a Volumio pi with a DAC board directly connected to speakers
Volumio has the built-in ability to start playing a playlist as an alarm, but not ramp the volume
HA can definitely control Volumio's volume so this seems like a good starting point.
I might even try when I'm home next.
This is pretty easy to DIY.
Esp32, PCM5102 DAC, and a small speaker.
ESPHome media player, the music either on Spotify via MusicAssistant or simply in a local folder on home assistant. You can adjust the speaker volume via the pcm5102.
Hell, wire an i2s microphone to it also and you have a voice assistant satellite.
I think you either need a music player that is on or on standby 24/7, or build one yourself. That can be done with a microcontroller plus other components, some soldering and the ESPhome project.
Supporting a product does this is one thing.
Making it work is another
One is easier than the other.