"My own late 2015 is still running 10.14.6 (great machine) that is why I bought another for the misses.lol"
OK, here's how I'd do it:
You will need:
1. An external USB3 drive. As SSD would make things go faster.
2. CarbonCopyCloner (FREE to download and use for 30 days, get it here)
www.bombich.com
What to do with this stuff:
1. Make sure the external drive is formatted to APFS, GUID partition format
2. On YOUR Mac, create an account for your wife, using the username and password she wants. It should be an administrative account. Aside: your wife may already have her own account. If so, you don't need to create another one! But... make sure it has administrative privileges, you can modify this in the "users & groups" preference pane!
3. Now, open CCC
4. For your source drive, pick YOUR Mac (of course)
5. For your destination drive, choose the external drive
6. "Safety Net" should be OFF
7. Where it says "Clone -- all files", you need to change to SOME files
8. Now you can selectively "un-check" stuff that you DON'T WANT to get copied.
9. YOU MUST BE CAREFUL HERE. You don't want to leave out files that are essential to the OS to run.
I would suggest that you go down into the "users" folder, and just uncheck YOUR account name. That excludes "your stuff", but it will let her new account "go over".
This is probably the only thing you need to change.
When you're done, click the clone button, follow through, and "let 'er go".
The clone will take a little while.
When done, power down, disconnect the external drive, and take it to your wife's new iMac.
Now, make sure the new iMac is POWERED OFF.
Connect the external drive.
Press the power on button and IMMEDIATELY hold down the option key, and KEEP HOLDING IT DOWN until the startup manager appears.
Select the external drive with the pointer and hit return.
The iMac should boot from the external drive.
Now, I'm guessing, but you should be offered YOUR WIFE'S account to log into (because you ERASED yours, remember?)
So, log in with her username and password from now on.
You should now be logged into her account.
Next, open Disk Utility.
Go to the "view" menu and choose "show all devices".
You should see "the top line" on the left representing the internal drive on the iMac (you didn't say WHAT KIND of drive it is -- HDD, SSD, or fusion?)
See if you can click "the top line".
Then, click "erase".
Choose APFS with GUID partition format.
Can you now erase the drive?
If so, this is what we want.
Once erased, the drive is empty. Now it's time to "clone over" the contents of the external drive to the internal drive.
Open CCC.
This time, the external drive is the source drive.
The INTERNAL drive is the DESTINATION drive.
Safety net can be off.
Chose "clone ALL files" (that's the default).
OK, let 'er go again. It will take a little time.
When done, the moment of truth.
Power down ALL THE WAY OFF.
DISCONNECT the external drive.
Press the power on button and use "the option key trick" as mentioned above.
You should get the startup manager and the internal drive.
So... select it and hit return.
Do you get "a good boot" to the login screen?
If so, you're almost done.
Get logged in.
Now go to the startup disk pref pane, click the lock, enter password, and click on the internal drive, then close system preferences.
Try the reboot again, this time DO NOT hold down the option key.
How do things go?
If it boots to the login screen, you're done!
