You can get Bluetooth receivers with stereo audio out for dirt cheap on Amazon.

A decent solution is to install shairport-sync on the Pi and advertise the service over multicast dns (Apple bonjour protocol). This effectively creates an AirPlay device on the network that’s usable from any iDevice. This had a very high “wife approval factor” when I did something similar at home.

https://docbot.onetwoseven.one/linux/airplay/

Sounds like there’s a headless version of Plexamp that can run on a Pi.

If you use Plex, you would be able to use Plexamp on your computer or phone to play your music on the Pi’s Plexamp.

Or you could get a modern receiver that supports Chromecast and AirPlay and use one of those to stream to the receiver.

There are bluetooth to AV "adapters", basically a small device that can be controlled from your phone like a bluetooth speaker and can be connected to your audio system.

WiiM

I've had some success using moOode audio on a Raspberry Pi (included in the Pi Installer I think, but here's their website in case I'm misremembering). I use it with Roon, which is linked to my streaming service and allows me to stream tracks; but I see it works with Spotify Connect too, so if that's your service of choice then perhaps this will work for you!