I have a similar setup at home: local AdGuardHome deployment with NextDNS as the upstream. I have several NextDNS configurations and AdGuardHome's upstream is one that has no filters. I use other profiles with filters when I am on the go.
I can't install NextDNS client on a lot of devices like a robot vacuum. Sure you can configure NextDNS to be the resolver on the gateway, but then the gateway becomes the only client in your NextDNS logs and you can't figure out which device is downloading Google ads, which makes the log useless. It is also easier to temporarily disable filters -- changing DNS on some routers may need a rebooting, which causes downtime on the whole home network.
The only thing you can do with NextDNS logs is looking at them on NextDNS web UI. I had a lot of fun messing with my local AdGuardHome logs with visualizations, analysis, and alerting.
I removed my PiHole once I got NextDNS as it just seemed redundant.