That curve for Defensemen falls off a lot more gradually than Forwards. Here it is with some proposed and current contracts by age to compare:
If these players follow the typical curve, Trouba would lose about 0.75 WAR over the course of an 8 year deal starting next season, since he's starting at the top though, he'll only be 0.75 off his peak. Karlsson would lose about 0.60 on an 8 year deal that starts in 2019-20. Buff will lose about 0.20 over the course of his current deal. Little can be expected to drop about 0.875 over his next deal and end up 1.375 off his peak. Wheeler will drop about 0.95 if he signs a 6 year deal to begin in 2019-20, ending up about 1.8 of his peak.
Conclusion: sign Karlsson, not Wheeler.