Is there another vet in your area? Your current one sounds rubbish honestly and if it were me I'd be trying to find a different one. When my now-deceased dog Bear had to have a few very invasive fatty lipomas removed last year and got a massive infection due to them accidentally giving me the wrong antibiotic all his follow up was free and they were extremely apologetic. (And to be clear, that isn't what caused his death. He recovered, lived another 7-8 months, and passed of something unrelated.)
I'm sorry you both had to go through all that though, but grateful it's over. And sorry about the cost, that really doesn't seem fair to you and it feels like a good vet would have handled it better.