andoff[1] works well. I use it to lock down DNS settings to nextdns to block all the sites I want. Then I use lockmeout[2] to lock opening andoff to change the settings.

Also there is limitphone[3], but it has less settings and is easier to uninstall than andoff, but works via the same mechanism.

1: https://docs.andoff.one/ 2: https://www.teqtic.com/lock-me-out 3: https://limitphone.com/