Random stranger sims entering home despite locked doors.

@GnatSexy That sounds very frustrating! In addition to the things you mentioned you could try...

1. See if there's any possibility that a NAP from Eco Lifestyle is influencing them.
2. Or if there may be an issue with a need for a game repair that's not completing properly. If you’re on Mac, the repair in Origin will only repair your base game. Learn why and how to repair your DLCs (Expansion Packs, Game Packs and Stuff Packs) in this article Bluebellflora posted on her website for great directions on doing this on Mac. https://bluebellflora.com/2020/06/17/repairing-sims-4-packs-in-origin/
3. I have had occasions in the past where a lock(s) acted up on me sometimes when I put multiple types of 'lock' conditions on it. You could try only adding one lock condition to all of the exterior doors to test to see if the issue still occurs (I would recommend locking to all but household members first). Unfortunately then you won't know if someone comes in due to vampire issues, but if someone comes in maybe you could look them up and see if they are a vampire? Also, if someone comes in the home make sure they don't have a friendship level with a household member that would explain it.
4. I'm assuming you don't have roommates on the lot? If you do, random people may come in that are invited by them.
   (This and some other reasons they may come in.)
5. If the above tests bring no answers, please see if the issue is present in a new game save.