Display Lock is not possible

[piero]

@Criguno

Do I need to take care for the Security Patch date when going back from e/os to Android?

no, only when you relock …
in the case, don’t relock FairphoneOS

@piero

Most important thing to take care about is the “android Security Update" aka "Security Patch Level” date (SPL) included in the already installed Operating System on your device at time of the unlock.

It must be OLDER

OR EGUAL

@piero

than the “android Security Update" aka "Security Patch Level” date (SPL) included in the /e/OS build you where you want to relock the bootloader, otherwise you risk to change your device into Brick.

@Criguno

I don’t like it to have so many different instructions spreaded over different locations.

official FairphoneOS and /e/OS instructions are very basic and can’t stand your situation differs than the default one.

the unofficial guide try to take in account maximum different device states…