You actually hurt yourself in 2 ways by paying on this old charge off.
The first is that you reaffirmed a debt that was close to the expiration of the statute of limitations in TX. If you would have waited for only a couple of months you would have been free and clear of being able to be successfully sued.
The second thing that you need to understand about credit is that the older a negative is, the less impact it has on your score, but when you have "activity" on the account, you make it more recent for scoring purposes.
Also note that paying on a chargeoff or even paying it off in full will NOT remove that item from your credit report, or help your score. Therefore, there was absolutely no reason for you to pay on this old account.
Also understand that the fact that Santander bought the defaulted loan does not restart the 7 year credit reporting SOL, nor does it restart the SOL for legal process.
And, forget the BBB! They have absolutely no power or authority to do anything to anyone. They are NOT any form of government agency or consumer protection organization. They ARE a "for profit" BUSINESS. They could care less about your problem. They only care about extorting membership fees from business owners in exchange for a "favorable" rating! The BBB is a corrupt and useless organization that is obsolete. And just the fact that you relied on them for help tells me that you are pretty clueless as to how things work in the real world. And, FYI..The FTC has no authority either in this case. Your complaint to them was just another waste of time and paper.
Also, after "charge off" interest is reduced as per federal law, and other fees and penalties stop. AND, it is ILLEGAL for "collection fees" to be added to your balance.
With all the damage you have done to your own situation, your ONLY option now is to get them in court and make them justify the balance owed, now that you re-affirmed it by making payments.
Knowledge is power, so you should ALWAYS educate yourself BEFORE taking any action that can hurt you.
You can do a dispute with the credit bureau for the re-aging of the account reporting. The negative reporting starts with the first major delinquency and ends 7 years after that date. This time starts only with the original creditor and never changes no matter how many times it is sold or assigned for collections.