If its any consolation, single rooted teeth usually respond the best to retreatment. I had a case a year or so ago where a friend of mine was quoted 7K for an implant to replace a failing RCT. We managed to save the tooth with a simple re treatment that cost less than a 10th of the implant cost. It was a gamble, but well worth it in that case.

Thermafil isnt necessarily a bad technique. Im not a huge fan of it but if the canal is sterile at the time of obturation (filling) it shouldnt matter what the canal is sealed with. Current research seems to point to success depending on how well the tooth is sealed at the top level rather than the root canal system. It still seems counter intuitive to me, but thems the facts from the researchers.

Never the less, redoing the root canal is often the better and less expensive choice than doing an implant. Endodontists often have access to microscopes to make sure there arent any hidden extra canals.

Fingers crossed for you, Im sure you will get a good result now.