Because ProtonVPN’s NetShield is sub-par at blocking ads whereas NextDNS uses custom blocklists and is far more reliable at this. The level of control you have using NextDNS over NetShield is night and day.
I have since realised that what I’m looking for is a more comprehensive custom DNS solution within ProtonVPN that gives users the option to use custom DNS over DoH and not limit it to IPv4 only (a bit ridiculous in 2025?)
This would enable me to use VPN and a well-configured DNS over DoH, then throw uBlock Lite for iOS on Safari and I think that pretty much gives full protection from ads on iOS at the system and browser level as well as a VPN connection. It’s the most complete solution I can think of.