What hardware are you using to Host? That'll influence the answer.
homeassistant
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
MA and HA are hosted on a RPi4 in the garage, not near the speakers.
Squeezelite with a USB spdif out might work? (I'm unsure myself, I have my squeeze server able to connect to my casting devices)
Bonus: a USB spdif out isn't pricey says a very quick search
Edit: I'm dumb. https://github.com/sle118/squeezelite-esp32?tab=readme-ov-file#spdif
So they suggest either a specialized chip, which will likely need more hardware, or this little circuit. (See link) I would say if you had an extra coax cable, to cut an end and solder away, but that's assuming a lot.
Second edit: they can connect to Bluetooth as well.
Thanks mate, much appreciated. That's what I mean, it's complicated... I have no idea what extra hardware I'd need. I just want to connect it via Coaxial and the only thing I can find is very complicated docs like you found :)
BT is not an option as I'd like to keep using the tablet directly to BT in case MA/HA is down.
You could try getting a Raspberry Pi Zero together with some kind of SPDIF output card, but that will probably go over $30.
I have no idea what pricing is like, but you could possibly try getting a used Logitech Squeezebox player.
If you're desperate to stay on the cheap and don't mind BT quality, you could also install Snapcast on an old phone, enable the Snapcast player provider and then use the phone to connect to your speakers over Bluetooth.