airplay 2 with (old) airport express

Since the introduction of Airport Express many years ago, I installed a number of them and I was able to stream music, via iTunes on my macbook, to several rooms simultaneously. (With the Remote app on my iphone I can also control the iTunes library of the macbook).

I would like to do the same with stream services such as Spotify, from both my macbook and iphone.

For the macbook, I installed Airfoil and this allows me to select multiple speakers (including my old Airport Expresses and a new Sonos Beam supporting Airplay 2).

But I cannot do the same from the iphone. Through the Sonos app, Spotify app or just iOS 11.4 on the iPhone I can only select one speaker at the time. I suspect that if I had more Airplay 2 enabled speakers (instead of the Airport Expresses) I would be able to select them for simultaneous listening. But it seems that the Airplay 2 feature which allows this in iOS 11.4, does not work with the old Airport Expresses. I find it strange though, because it has always been possible (and still is) to do it in the macbook (either via iTunes or Airfoil).

My question is therefore: On the iPhone, how can I transmit audio from e.g. Spotify to a pool of speakers, including Airplay 2 speakers like Sonos Beam AND speakers connected to Airport Express.