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Flashing must be enabled in the OS settings. Without that, the bootloader can't be unlocked.

Yes sorry forgot that. First thing to do when buying a phone is to enable Developer settings and unlock bootloader I guess

You could but then you also leave it wide open for others to mess with it if they get their hands on it (evil maid attack)

There's a reason security-conscious roms like GrapheneOS and CalyxOS only support devices where the bootloader can be relocked after installing.

there also quite a few apps, like many banking apps that refuse to work on a rooted phone.

These roms are not rooted as they consider rooting a security hole (which it is if you don't know what you're doing)