iPhones are a fine choice for a lot of people. They're reasonably secure (more than most Android phones, really). If GrapheneOS didn't exist, I'd probably be using an iPhone.
That said, having used GrapheneOS, I personally don't think I could use anything else at this time, there's too many GrapheneOS-specific things that I would miss.
A lot of the stuff you mentioned like private relay or hide my email, these are Apple services, but they exist outside of that. E-mail alias services like SimpleLogin or AnonAddy, along with VPNs that have multi-hop features. Apple's really solid in a lot of ways, but the thing they do best is marketing. They name things differently to make people think their offerings are one of a kind, when that's not always the case! :)
Lockdown mode is a funny example actually. It really does a lot, but the way it's implemented is problematic. It's a toggle that does a whole bunch of stuff, and if you're unhappy with some of what it does, you have to forego the numerous benefits it provides. As a result, it's also not enabled by default, which is a shame. It's also important to note that a lot of what lockdown mode does is disable Apple services for attack surface reduction (the same services most people flock to an iPhone for :))
In contrast, GrapheneOS always prioritizes mitigations and security/privacy features by default, not disabled by default and behind a binary on/off toggle.
https://grapheneos.org/features#exploit-protection is just some of the things it provides by default, along with default-disabled JIT in the browser etc.