What if I am approved for TDIU, but the VA has established my effective date as the date I filed my TDIU application, but in reality, I was unemployable long before that date?

Can I appeal the effective date only?

Yes. Case law has established that the proper effective date for a TDIU claim is the date the Veteran became unemployable, not the date the application for TDIU was filed.  This is a common VA error that can be corrected on appeal or with a motion for revision based on clear and unmistakable error (CUE).