You can setup your WiiMs as a Speaker Group in the Google Home app, and then Google Cast directly to that speaker group in SiriusXM app. Works well. For me it works much better than using AirPlay (as AirPlay from iOS will eventually stop playing because the device you cast from eventually loses its connection).
My favorite way to listen to SiriusXM though is on Apple TV, and then use Airplay in Control Center to play to your other Airplay speakers (Sonos, WiiMs, etc.) simultaneously. You won’t lose your Airplay connection in this scenario because Apple TV is the source playing it (which is always ‘active’). You also get the nice UI from SiriusXM app to show what’s playing on your TV (I like it better than the new WiiM Home app). But I love the new WiiM Home app cause that’s good to use for all your other non-SiriusXM sources playing on WiiMs.