i loved that dog but for her sake i do wonder if she would have been better off had someone with money came along and taken her

I don't think this is a healthy way to think about your dog's misfortune. There's no way "having the resources to pay for possible future cancer treatment" is a reasonable prerequisite for taking on responsibility for a dog. You minimised her suffering and that is sufficient. You rescued her and you didn't let selfish irrationality stop you from making a hard choice that she needed you to make.

You were the one who stepped up for her, twice. Not some hypothetical person with more resources.

I'm sorry for your loss. It's gut wrenching.