A quick glance at NextDNS' web page shows it is a paid service. Both Pi Hole and AdGuard Home provide the same thing, but installed locally. You can point the upstream DNS in both Pi Hole and AdGuard to any DNS server you want, but the ad blocking is done locally, and domain lookups are cached locally, which offers a faster response time. I've been spoiled, as I've always used a locally installed DNS on my networks [usually TinyDNS aka djbdns]. A few years ago I migrated my home network to a combination of the 2, as I use AdGuard Home as my DNS server that all my clients are pointed to, as that does the ad blocking and uses another physical server running TinyDNS to do the "upstream" lookups. Most people will probably use 1.1.1.1 as the upstream, but I like having more control, and being able to do encrypted DNS helps keep the MIM DNS attacks that have recently plagued macOS and Windows [even when people are using 1.1.1.1 - there's a great article at https://apple.news/AN04Ij3FwSua6UqhRhZCNjg which talks about how an unmanned ISP was hacked an updates that OS' do in the background became comprised - another ISP is also having the issue but has not yet been corrected].Do these ad-blockers conflict when installed on the Mac? While Adguard seems quite effective for Safari M1 Pro, along with the DNS server 1.1.1.1 setting, NextDNS (the app) sounds like an excellent approach, according to the above postings. Would these two apps work well together or better off staying with one or the other? I assume NextDNS would reset the DNS server to other than 1.1.1.1.
It is my understanding AdGuard's premium product takes the place of NextDNS as they do their own DNS as well. I again, just use their free products, paired with the User Script Extras and their open source AdGuard Home installed on a Raspberry Pi. I use https://apps.apple.com/us/app/adguard-home/id1667119601 on my iPhone/iPad to administer AdGuard Home but you can also use Safari as AdGuard Home has a nice web interface. AdGuard Home does more than just block ads - you can restrict access to certain websites, etc.
NextDNS is a paid [after a certain number of queries] DNS service that would replace any external DNS you have your clients pointing to. If you use 1.1.1.1, and are happy with it, then I would keep that. But if you're looking to block ads for free, using AdGuard Home or Pi Hole, and 1.1.1.1 as the upstream DNS defined in Pi Hole's or AdGuard Home's config, would be the best of both worlds.