CrJ I really don't like changing the GrapheneOS settings like this so I'm hoping someone comments with solid info on privacy/security related to this change.
Specifically for the three carrier overrides mentioned, there are no security or privacy implications.
The worst they can do is intefere with getting the best (or any) mobile connection. Most of the time any adverse effect can be reversed by disabling all carrier overrides and rebooting the phone. Occasionally, it might be necessary to "Reset mobile network settings" (DO NOT check the box, DO NOT erase SIM) and update APN settings (if you had to do that previously).
What you have done before is roughly the proper way to go about it, i.e. don't play with the 5G overrides until you get things reliably working on 4G first.