Setting Datum Layers after the Models are Created
Solution #1
Have you thought about creating a trial file or mapkeys?
The best for large quantities is a trail file.
1. Update the first part exactly how you want to see it. Do not use a mapkey,
Pick thru.
2. Go to you trail directory and Copy that trail file to a new name.
3. Open the trail file
4. Do NOT delete the first 3 lines (this is coded)
5. Delete every thing except from: File, Open to Window, Close
6. Copy these lines and paste below with no spaces.
7. Replace the Part name.
Try this for 2 parts then expand to more parts as you get the hang of it.
Doing trail files is a feather in your hat.
Solution #2
First off, the problem can be avoided going forward by changing our start part and placing "set datums" on a default layer.
Go to the top-level assembly and do it there. Pro will prompt to create a layer in a dependant part if you try and manipulate it from the top level.