Have you thought of a Livestream app? Multiple apps specialise in music hosting, some of which even working with streaming services like Youtube and Spotify. Some that come to mind are TheClub and Ampme.
If you're still set on going the Bluetooth route, Cassia makes a Bluetooth router that can connect to up to 22 devices within a 1000ft radius.
https://www.amazon.com/Cassia-Networks-C100-10-Bluetooth-Router/dp/B01M1H55DO
I don't think the TheClub app stuff would work as it was something I originally recommended but got shotdown. The Cassia looks nice but how much and real technical overkill? Can't find a price anywhere and that model is discontinued.
~The Cassia C100 retails for roughly $100.~ Avoid used as product is no longer supported.
Thonet&Vander has secured the rights to this product and sells it via their website for $150. You can purchase one at https://thonet-vanderspeakers.store/thvstores/product/multi-room-hub/
Not available anymore.
Answer from the seller in reviews:
The Cassia Hub Bluetooth Router is currently only compatible with the ECO dot for playing music to your Bluetooth speakers at a distance. The ECO dot connects to the Cassia Hub which in turns connects up to 3-Bluetooth speakers or connected speaker pairs. If you have any additional questions, please feel free to contact Cassia Networks
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