So, if you were Dem leadership, what would you be "whipping" your members to do?

They're whipping them to not vote for the bill. That's it.

If Schumer wanted to give Thune 8 votes to pass the CR, it would be a simple ask - just like in March. They only need to whip the members to prevent another handful from doing it on their own.

There's not a lot of deep procedural strategy here.

The Democrats are picking this fight because their base wants to see that the leadership will refuse to do something the GOP wants without getting something in return. Sure, they've picked "ACA subsidies" as the thing to ask for; but mostly because that's something that plays to one of the few issues where the Democrats are preferred by voters.

Unless the Reps actually relent, which I don't see them doing (they have always hated the ACA, even though many of their constituents benefit from it).

As was pointed out earlier, the Dems insistence on the subsidies being extended actually solves a problem for the GOP. They don't want to take the hit for the subsidies expiring. But the Freedom Caucus wing of the party is solidly against them. Johnson couldn't pass a bill to extend them through the House without getting help from the Democrats. He wouldn't normally be able to get an extension of the subsidies through the House without giving the Democrats something in exchange for their votes... except now, since the Democrats are so on the record on the subsidies, if Johnson brought a clean extension to the floor there's no way they could ever vote against. They're using up all their leverage on that vote in order to bargain for...just ending the shutdown.