It's an impossible line to draw, and the only people who could draw it are NCAA enforcement personnel who rightly have a public trust level of zero, even if the courts ever allow them to have jurisdiction again.
Because it really isn't just "national" deals. The funny De'Coldest Crawford local Nebraska HVAC commercial thing is very much in the spirit of "true" NIL also. Plus players selling t-shirts on Instagram or whatever. There's no legal or moral justification for stopping those activities. But there's no line to draw between that and a booster just handing over a sack with a dollar sign on it, which of course was already happening anyway, it's only the legit deals that were being stopped.
The only legally workable way for the NCAA to restrain the trade of its labor is the same way the pro leagues do: to collectively bargain with them.
That sounds easier than it is though, for a panoply of reasons.