There is duplicate logic in both x/tx/decode and the x/auth/tx packages. Since the decision to support gogoproto throughout the SDK (as specified in core/transaction below)

These two packages can be simplified and collapsed into one simple implementation in x/tx. An initial exploration of this was done here in #20424 and integrated in #20428.

The result will be a lot of deleted code and much less mental overhead when reasoning about transactions in the SDK. The signing code in x/tx can and should remain the same, as shown in #20424 reflection over gogotypes as input is possible through the use of dynamic messages.