You'd need to always keep your phone in airplane mode, and maybe either use a mobile provider that works horrible with GrapheneOS, or change your mobile strength to like 3G, you can completely lose calls and texts when in airplane mode at that point, you'll never get them, maybe you can stop the Pegasus attack.

Or, don't use a phone number, rely on encrypted messaging. But if you must have some number, you could have 2 phones, one with just the phone number and sim card, then the other phone with Signal so your private phone shouldn't be compromised.

Those are the only ways I can think of trying to resist it.

That's true, all devices are hackable, there's no 100% protection.
No tool is perfect, but if that's a security improvement, it might be worth enabling.

I know of at least one instance where lockdown mode protected a user from NSO spyware.
A Citizen Lab's research confirmed it:

For a brief period, targets that had enabled iOS 16’s Lockdown Mode feature received real-time warnings when PWNYOURHOME exploitation was attempted against their devices. Although NSO Group may have later devised a workaround for this real-time warning, we have not seen PWNYOURHOME successfully used against any devices on which Lockdown Mode is enabled.

It is encouraging to see that Apple’s Lockdown Mode notified targets of in-the-wild attacks. While any one security measure is unlikely to blunt all targeted spyware attacks, and security is a multi-faceted problem, we believe this case highlights the value of enabling this feature for high-risk users that may be targeted because of who they are or what they do.