Yes. If you don’t connect it is pretty dumb and shouldn’t be able to send your data for harvesting. You sould research if you can set it to one of the outputs permanently then you can use some external device that you trust.

Yes and no. For apple you can use their phones for quite a long time securely. For Android that is a very different story. As far as I know only Google with their new pixel phones and Samsung have offered more than 2 years of updates. After that time your phone becomes a security risk. So make sure your devices receives updates or can be used with a custom ROM (though that can be insecure as well).