I personally wouldn't be joining MilleMiglia at this time. It is complete unknown what will happen to it and any points you earn in it. If you are looking to fly with ST airlines then join one of the other ST programmes (not CO since they're leaving ST).
If however you will be planning to fly mainly the new AZ then perhaps still evaluate MilleMiglia, but I'd be very careful, as you have no idea what the earn/burn ratio will be like after the takeover is complete or if MilleMiglia will even exist. There are a lot of unknowns and that doesn't make your miles particularly safe, I'd personally prefer my miles to be somewhere a bit safer. Better the devil you know than the devil you don't know
