OP:
Here's a pathway forward for you, using CarbonCopyCloner.
PRINT THIS OUT and check off each line as you go along.
Boot up the OLD iMac for now.
Can you get to the finder without problems?
If so, do this next:
Open the Users & Groups preference pane
Click the lock on the lower left and enter your password.
Now, click the "+" icon to CREATE A NEW USER ACCOUNT
MAKE SURE you also give it administrative privileges (popup menu).
Give it a name and password for your husband, and WRITE THE PASSWORD DOWN ON PAPER for your husband.
Now, create this account.
Now, log out of your existing account and log into the NEW account.
Now, connect your SSD.
Open disk utility and erase it to APFS with GUID partition format
Does it erase ok?
If so, let's go on to the next step.
Open CCC.
Accept the defaults for the moment
On the left, put the source drive (iMac internal drive)
In the middle put the destination drive (SSD)
IGNORE the "box on the right" (scheduling)
BUT DON'T CLICK THE CLONE BUTTON YET !!!!
Now, we have some "selecting" to do.
In the lower left of the CCC window, there is a popup that will (by default) be "copy all files".
We need to change this to "copy SOME files".
Now, a new window will appear with a bunch of checkboxes.
This is how you can EXCLUDE files from getting copied.
This is how we'll instruct CCC to clone only the basic OS, apps, and the new user account to the new SSD.
Now, I can't see YOUR drive, so I don't know all the folders that might be at the top level of your drive.
Nor do I know where you keep your data files.
Do you normally keep all that stuff inside your home folder?
What you need to do is to go down to the "users" folder.
Click the little "disclosure arrow" and it will reveal what's inside.
You want to UNCHECK EVERYTHING EXCEPT the new, basic user folder that you just created for your husband above.
You also need to uncheck ANY NEW FOLDERS THAT YOU CREATED "at the top level".
You want to leave everything else UNchecked -- these are the visible and invisible files that comprise the OS and are needed to run the iMac.
We are doing all this so that when the clone runs, it will copy only the OS files, the apps, and the new basic account for your husband. EVERYTHING ELSE will be "left behind".
Once you've got this done, click the "done" button to close this window.
Now click "clone", and "run now" when prompted.
Then, sit back and wait awhile.
How does it go?
Does the "selectively slimmed down" clone "go through" ok ???
If so, when done, quit CCC.
Now POWER DOWN the iMac, all the way off.
Wait a few seconds and press the power-on button and IMMEDIATELY hold down the option key and KEEP HOLDING IT DOWN until the startup manager appears.
Do you see the external SSD?
If so, select it with the pointer and hit return.
Do you get "a good boot"?
If so, you win!
