The problem with relying on the 8th alone is that the word "excessive" is imprecise, and there's good reason for the Supreme Court to defer to local legislatures to define what it means, except in extreme cases. Deciding criminal penalties are normally within the powers of the state.
I think the more important amendment is the 5th: ("nor shall private property be taken for public use, without just compensation."). This "takings" clause is what civil forfeiture is all about.
The 5th Amendment has been fully incorporated, and is binding on the states: https://www.law.cornell.edu/wex/incorporation_doctrine
In cases where there is no rational connection to a crime (and there are many such cases), the taking can in no way be construed as a fine.
If you look at the 5th and the 8th together to try to divine the intent of the founders, it's clear that they were trying to limit government overreach of exactly this kind. This case should be a slam dunk.