Do other times you want separate channel control? If not you might switch the shelly you're using and control on the high side versus low?
I don't know about Alexa, but with Siri (running via Hoobs) I could put the device in a separate room and then tell siri to turn off that room. For example I have an RGBW2 controlling 2 strips and 2 fans (plus a different power strip with things) I can say "Hey Siri turn off the greenhouse" and it shuts everything off.