Bluetooth speaker for airbnb?

Apple home pod

Does it have to be mountable? If so, I think the easiest thing would be to just get a soundbar of whichever brand you want. If you are putting them in rooms with tvs, you can use them for that as well.

Denon NV150. Spotify connects directly to it. And you're not just looking necessarily for a Bluetooth speaker, just connect the speakers to the same Wifi as the guests will use.

Soundbars suck compared to individual $180 speakers like the Denon and Sonos unless you spend a good penny on it. Lack of bass on soundbars without an additional sub, etc.

Yes sir... They are awesome and sound amazing

Just upgrade soundbar to one with airplay(and/or cast). Problem solved. Put an airplay sticker/sign on/by it so they know they can stream to it. Add it to the guest WiFi network.
Anything else would likely need instructions/support/whatever and be more likely to be tampered with since it isn’t a natural part of the home.
You can also set alerts when it is disconnected from WiFi so you know who to charge if it comes up missing.

Bluetooth is designed for portable devices primarily controlled by a limited number of other portable devices that remain in close proximity after a cumbersome pairing sequence. Bluetooth products are therefore typically designed keeping this in mind.
AirPlay/cast products are stationary devices typically meant to be immediately accessible by any device on the same subnet, regardless or whether or not they leave the room. These are much better suited to the OP’s use case.