Hi and welcome, just a side note: be lucky its so easily possible to unlock the bootloader (thats not the standard!) and relock the bootloader. All phones nowadays have a locked bootloader. You are concerned about security, so another point why you can be happy about the fact that the bootloader is locked. Also it is not a good idea to refuse updates when you are concerned about safety and security.
Have you done some reading/search in the Forum, this could give some answers already.
If you want a FP4 running e/OS without the need to unlock the bootloader, buy one pre-installed from Murena.