How do I enable Safe Boot if File Vault is enabled/

I just keep holding down the Shift key after entering the password.

FileVault2 has no impact on booting in Safe Mode / Safe Boot. You just hold the Shift key while powering on. Let go, authenticate and hold Shift again as fast as you can after hitting Return. You will see a second login prompt, let go of Shift and enter the username and password then again, press return and resume holding Shift as quickly as possible. Once you get to the Desktop let go of Shift. You will know you are in Safe Mode if the dock transparency is gone and it looks a bit off. You should not see any 3rd party add-ons loaded nor any kernel / system extensions.

Safe Mode / Boot will also run a bunch of maintenance such as flushing caches and the like which can resolve bizarre issues.

You might just need to hold shift till you reach the first login prompt and then won't need to continue holding shift but I always held shift all the way through to ensure no startup items launch, etc.

On an M1 Mac the process is different. Shutdown completely, hold the Power button and when you reach the Startup Options screen hold shift and click Continue in Safe Mode.

Is one asking a question only for the purpose of asking or is there are technical reason why one wants to disable FileVault when booting into Safe Mode.

From what is supplied by Apple as per below, booting into Safe Mode with FileVault will have zero affect on achieving a Safe Boot.

Safe mode may help you resolve or isolate problems that you’re having with your Mac.

When you start up in safe mode, your Mac prevents some software, such as startup items, from loading, and it performs a check of your startup disk. Your Mac may take longer to start up because of the check.

Try immediately after powering up; well before you get to the login screen.

Read here: https://discussions.apple.com/thread/253382887