Privacy isn't binary.
LineageOS without Gapps won't send information to Google unless you install something that does. It won't do a whole lot to prevent apps from collecting data like GrapheneOS does so it's up to you to evaluate the privacy implications of anything you install.
A locked bootloader protects against two attack vectors: malware modifying the operating system at runtime, and an unauthorized person with physical access installing a malicious operating system while you're not looking (an "evil maid" attack). The former is rare on Android. The latter is rare unless you're a high-value target or dating an abusive hacker.