Trimethoprim sulfamethoxazole (aka. Bactrim, tribrissen, TMP-SMX) can be injected or given ORALLY, kills Streptococcus and it´s safe for pigs, and the dosage is 15-30mg/kg twice a day. I´m not a vet, but found that on the web, so get your vet to check it´s correct. Some antibiotics are more effective if injected, I don´t know if this is one of them, but it is better orally than none at all I´m sure, and less traumatic for the pigs. It might be worth continuing to inject the worse ones for this reason.
It must be hard to inject so many pigs, and not knowing if it´s going to work. Just think if it saves just a few and gives them a chance at a full healthy life, it´s worth it.
I really feel for you and hope that they will be ok, try not to give up hope, try to think about the ones you are keeping alive. So sorry, that you and your pigs are going through this.