Have you considered engaging the services of a CDN such as Cloudflare that specializes in such mitigations? Rolling anything on your own to mitigate this level of traffic is going to be a large undertaking and not something you're going to be able to do whilst under attack.
Since from your post this looks to be a layer 7 attack your options would either be putting your service behind Cloudfront or Cloudflare and using their respective ddos mitigation tools. They also can provide support to get things configured and working effectively.
There are other similar solutions out there that I've not had experience with so can't comment on, but utilizing one of the hyperscale services will be your best bet.
Since from your post this looks to be a layer 7 attack your options would either be putting your service behind Cloudfront or Cloudflare and using their respective ddos mitigation tools. They also can provide support to get things configured and working effectively.
There are other similar solutions out there that I've not had experience with so can't comment on, but utilizing one of the hyperscale services will be your best bet.