I haven't really used adguard or nextdns before so I can't compare apples to apples. I can say that Rethink is a FOSS local-VPN-based adblocker that doesn't need root. I used to use a different VPN based one before that I forgot the name of, but because it was a VPN I couldn't also connect to my home Wireguard VPN at the same time, so I was swapping VPNs all the time. I like it because I can be connected to my home VPN, and then if that connection fails it automatically uses the on-device DNS blocklists, which can be customized which lists to use. It can also set different DNS rules / bypass filtering on a per-app basis instead of being forced to being system wide. It's been useful to allowlist certain domains for specific apps only to let them work.