My site is currently offline from this attack. I am considering that providers like DNSimple simply cannot provide the networking availability to mitigate these kinds of attacks. This is because the solution to these kinds of attacks, often, is that you need a larger pipe than the attackers. Very few people are good at both <some service> and having a huge pipe. For web apps, you can use CloudFlare, which does have a bigger pipe and is designed to mitigate this. But DNS is not a web app, so you can't just put DNSimple behind CloudFlare.

Hence, I'm going to try CloudFlare (assuming they take over DNS hosting, I need to check) and Google Cloud DNS, because then all parts of my site (from DNS to CSS hosting) will be with providers with bigger pipes than attackers can create. Hopefully that will prevent this kind of attack from taking my site down.