Solved! Updating my own post in case it saves anyone else hours on the phone to Sonos support. Here are my steps to resolve (not sure if all were needed, but this is what ended up working for me):
1. Delete and reinstall Sonos App on phone (Android)
2. Factory reset of Sonos Amp (hold down 'infinity' symbol on back while plugging in power, until light on front goes orange)
3. In the app, accept prompt to enable Bluetooth and Location. Follow prompts to add new system - the Sonos app should discover the Amp via Bluetooth
4. When prompted, enter the password for your WiFi router
I also was asked if I wanted to upgrade the firmware on the Amp, I selected "yes" here. And that should be it - after this your Sonos Amp should be joined to your local WiFi, meaning you can stream to it from your phone, just like before the upgrade.
Disappointing that it took so long to resolve and that Sonos made it so difficult - at $1000 I have to say I'd seriously have to think about buying another one :(