I currently use Secureblue, silverblue-main-userns-hardened to be precise. Mainly due to it's very good browser security because of the hardened-chromium subproject, which allows to disable extension support and has built in ad-blocking via the subresource filter.
I plan to switch to either ChromeOS or MacOS in the future, still not sure which one is the better option, even though I read the entire Platform security PDF from apple and the ChromeOS whitepaper. With ChromeOS specifically, it seems that it's slowly moving into being a budget-only offering, as good hardware for it is very hard to find and oftentimes several years old. MacOS doesn't sandbox all apps that aren't from the App Store.
Ultimately if we only include MacOS in Lockdown mose, there aren't any reports for either being infected with malware that I'm aware of, Google even states that on it's homepage for Chromebooks. Therefore both would be great options, but can't compare to GrapheneOS in desktop mode, which should be available soon. Of course more complicated workflows like application development won't be possible on it, at least not yet, but most everyday tasks should be great on it.