While I agree with you personally, I think legally that would be very far from a slam dunk case.
The situation in this case seems similar to the one in Bennis v. Michigan, where SCOTUS already ruled that the civil forfeiture did not violate the takings clause of the 5th amendment. So the 5A angle seems like a risky argument.
On the other hand, the waters surrounding the 8th amendment are very murky. As recounted in the Indiana SC decision, their basis for determining that the excessive fines clause is unincorporated comes from the 2010 McDonald decision, a case that had nothing to do with the 8th amendment at all. It was only mentioned in passing, so this was not a holding but merely dictum. SCOTUS had previously said in dictum that the excessive fines clause was incorporated. So there hasn't been a full holding, plus two contradictory dictums in a relatively short period. That's fairly murky, whereas Bennis vs Michigan is basically clear with regards to the 5th amendment.