I think NextDNS is perfect but there is one thing id love to see.
As an I.T security guy, I have been using Hosts files since the start of the Net so I never used an Antivirus as they are all scams, I know this Cuz I used to make viruses for these companies. Anyway I would love to see what I would call a List Master with is simply a text doc server side that you can copy and paste large lists of your own in, plus you could have a duplicate remover and be able to change the 0.0.0.0 to 127.0.0.0 if you wish.
It is an excellent idea because at this time you can only put one URL in the deny list at a time with is very annoying, so at the moment I use NextDNS from my router but I still have a personal hosts file for each PC I use to block more windows and apple URL's, plus the list goes on.
Maybe you could do it for paying customers only as I do not see any other DNS companies doing this excellent text editor idea.