NextDNS integration with custom DNS over HTTPS doesn't work. This is because DNSnet doesn't accept this entry:
https://dns.nextdns.io/123abc.
And since it doesn't integrate TLS, 123abc.dns.nextdns.io doesn't work either.
123abc is an example because NextDNS allows its members to customize their queries. I look forward to your feedback and thank you for your work so far.