Hello, I have good experience dealing with DDoS attack, Few month back I faced similar problem of DDoS on my aws servers, We also tried all things like CloudFront, country blocking etc, but nothing worked. Later we found out there is a exploit which hackers use to easily DDoS server, no mater how much bigger server you have or which DDoS protection service you use, the exploit let them DDoS server easily. Later we fixed the exploit which stopped all DDoS attacks on us. I think I can help you preventing DDoS on your servers, as I learned many things during that incident.