Secure from cyberattack

Nah, NextDNS only handles potentially malicious domains. It can't help with websites and apps exploiting vulnerabilities in Android itself. If your phone doesn't support custom ROM and your account is used for financial services (even if it's not in the phone itself, since if your phone is hijacked they can use your email to reset passwords), just buy a new phone. Low-end Samsung & Nokia cost less than $100 unlocked.