rethinkdns co-developer here
If you’re on Android 10+, you can set NextDNS as a Private DNS provider on your Android and use NetGuard concurrently with it.
Regarding the app RethinkDNS+Firewall, you can use the app to point it to any DNS resolver of your choice. You’re not beholden to use RethinkDNS’ own DNS resolver.
If you are concerned about its privacy policy, then see: Privacy Policy | Rethink
NextDNS Android app isn’t open source.
NextDNS the resolver isn’t open source.
And if something isn’t on F-Droid, then it might be prudent to ask the developer about it rather than the F-Droid community, though the volunteers and the community do tend to invest a lot of their own time and effort helping FOSS developers onboard.