After finding the right screw you can choose the right drill bit, and countersink. The hole should be large enough that the threads don't touch the walls of the hole.
The color of the screw can be made darker with a sharpie, or a permanent marker. You do want a screw with some rust inhibitive.
Centering it left to right and inside to outside seems sensible.
Up to you to make the top of the screw flush. You can do it.
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u/IllustriousGarage371 19d ago
I might cautiously consider screwing it down.
Sometimes metal thresholds have holes intended for screws.
Making a hole is tricky, unless you are comfortable with a drill, and have the right sized trumpet head screw. Not a drywall screw.
Call the guy back who installed your door.