Using a private DNS with a VPN can be counterproductive, especially if using the DNS for ad-/tracker blocking. This is because when using a VPN one goal is to blend in as much as much as possible. This is achieved when a lot of users have the same IP address as well as DNS, so it’s harder to connect to a specific person. When using a private DNS this overrides the VPN DNS resolver. Therefore you might stand out from the mass making it easier to identify and track. There is also further explanation here at privacy guides.
(if I’m wrong about this than I’d gladly be corrected by someone with more knowledge)
Mullvad has ad-blocking DNS, even if it’s not as customizable, so might be worth using that instead with the VPN.
Depending on your use case and threat model you can choose your setup. I’d go with either Nextdns or Mullvad, not both, at least not at the same time. I myself use ProtonVPN on my phone, which also has ad-blocking. On my home network I have nextdns on the router (solely for ad-blocking) since I have a very privacy friendly ISP.
Edit: some clarification