Is this enough

To get both of them with retained salary? I can't imagine so.

What would need to be added?

Look at it this way....

Ekholm is a legit top four LHD that can play effectively in all situations. (Arguably even a top pairing one).
Forsberg is a legit goal scoring top line winger who can play effectively in all situations.

Both have the remainder of this year and next year left on their contracts at very reasonable numbers.

What would you want in return if the ask was reversed and it was Morgan Rielly and William Nylander going out the door with one year left on their current deals?

My best guess would be A LOT!

Expect nothing less from Nashville. (Especially if there are large retentions in play).

That is a great question. I'm no Nashville fan, but I'd imagine if they were packaging their two best assets with that much retained salary they'd probably want Sandin. He's got a higher ceiling than Liljegren. But some TSN people have said that he's close to untouchable, so I'm not sure.
But my guess is Sandin has to factor in there.

Robertson + Sandin + 1st round pick probably gets them listening. Anything short of that with those retentions.... I doubt it.