you’d need to build and sign the image yourself then before relocking. See the payton links at https://hub.libranet.de/wiki/and-priv-sec/wiki/verified-boot#Motorola
The benefit after locking a user-signed boot chain: instead of an orange warning for an unlocked devices, you’ll see the yellow warning. Not the award one is hoping for I guess, but the verified boot chain is an achievement.
For cosmetics, on my Motorola G4 with no implemented android-verified-boot (AVB) I changed the splash screens to what I liked - [HOWTO] Moto: replace the startup screen warning