I think that error message is a result of UndecidableInstances masking the real problem. Without that option, GHC throws an error, that it can not see, that the functional dependency t → r is satisfied in the second instance:

    • Illegal instance declaration for ‘Variadic a r (a' -> t)’
        The coverage condition fails in class ‘Variadic’
          for functional dependency: ‘t -> r’
        Reason: lhs type ‘a' -> t’ does not determine rhs type ‘r’
        Un-determined variable: r
        Using UndecidableInstances might help
    • In the instance declaration for ‘Variadic a r (a' -> t)’
   |
13 | instance (a ~ a', Variadic a r t) => Variadic a r (a' -> t) where
   |  

Removing the functional dependency resolves that, but then GHC complains about an ambiguous type in polyList.
I have no idea how to resolve that. I have been banging my head against the wall for a whole day trying to get a similar solution working, before settling for a less elegant solution.

As mentioned in initial code, the combinators are already defined in Preloaded. They should be used instead of defining your own.