why would anyone think they could open up a few more fronts against well armed NATO countries?

Russia could do this. It would just suck really hard for everyone, including Russia. Dictators who are currently running their country into the ground often do really, really stuipid shit.

The response to these incursions are to give Ukraine the capabilities to destroy Russian war production. Upset by Russian drone incursions into NATO countries? Give Ukraine long-range cruise missiles and let them blow up Russian drone factories. Russia understands strength, the course of action is to respond with strength.