remove any hinting of the provider (remove the text/logo NextDNS) to slow circumventing research by the naive.
I understand requests for custom blocking images would quickly become a resource hog. Alternatively allow paid users to provide a cloud hosted URL like BackBlaze or Accelerated S3 or cloudfront. Yes, pay yer own way.
Romain Cointepas simple logo and text. Nothing more complex.... Please see what open DNS offer of you were able to offer a custom page that would be better, but I would be happy with the basics at least