(I've never heard of them; this is a fundamental problem with using centralized "privacy preserving" services.)
https://en.wikipedia.org/wiki/Watering_hole_attack
I think ublock origin is a bit better, in that it is open source. Does it support reproducible builds though?
Besides, I don't know what a paying-customer relationship has to do with trust. I could just as easily be betrayed by someone I'm paying. For example, GrubHub drivers have a chronic problem of "losing" my drinks, despite being promised a tip and wages. I have to go chase refunds every day for these "mistakes". I'm a paying customer, yet I can't trust them to get my order right.
https://github.com/orgs/nextdns/repositories?type=all
https://github.com/AdguardTeam/AdGuardSDNSFilter
If you don't trust their DNS servers for whatever reason, you can simply add these entries to your hosts file to replicate their functionality locally.
If you want to have all the data under you control, there's this: https://github.com/AdguardTeam/AdGuardHome
Regarding open source, AdGuard DNS actually is: https://github.com/AdguardTeam/AdGuardDNS
In the case of AdGuard DNS being open source does not change the fact that it is a centralized service and using such a service is a matter of trust.